Battery overcharging

PusherJuly 30, 2013

Looking for assistance regardnig a Briggs and Stratton 422707 1214 01 lawn trractor engine. About three weeks ago I mowed the lawn and shut the tractor off. When I restarted the tractor about 15 minutes later the battery exploded (took the top right off of it). The battery was about 3 years old with no prior starting or running issues. After cleaning the tractor and replacing the battery, I placed a volt meter on it and recorded about 14 volts at the battery with the tractor running full speed. Being curious about what caused the battery to explode, I checked the voltage again last night and received a reading of 16 volts at the battery. I suspect the voltage regulator might be shot, however, could not locate one on the engine. The parts diagram lists a voltage regulator but I can't locate it.

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What's the brand & model# of the tractor?
maybe there's a schematic available so we can tell which of the various Briggs charging systems you have.

    Bookmark   July 30, 2013 at 11:21PM
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Thanks for the quick reply. The tractor is a Ranch King/MTD 18 HP Model 143P849H205 1C053810007. It appears that there are two wires from the stator (red and yellow).

This post was edited by Pusher on Wed, Jul 31, 13 at 11:01

    Bookmark   July 31, 2013 at 10:48AM
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***" It appears that there are two wires from the stator (red and yellow)."***
Better double check the colors of the WIRES and the plastic CONNECTORS.
Briggs has specific colors of WIRES & CONNECTORS to identify the exact (or nearly exact) system configuration/type.
Your statement about the stator having 1 red and 1 yellow wire is not exactly correct.
The component you are referring to that has 1 red wire with a red connector, and 1 yellow wire with a green connector..................actually is the regulator, not the stator.
The stator is under the flywheel. The yellow wire with the green connector should join to a black wire leading to the stator. The red wire with red connector is the charging output circuit that joins to a battery positive circuit on the chassis harness.
Your Briggs charging system is the "5 or 9 amp, regulated" system.
Whether it is 5 amp or 9 amp is determined by the size of the magnets on the flywheel. 5 amp has small magnets and 9 amp has large magnets.
The Briggs part number regulator you need is 691188.

    Bookmark   July 31, 2013 at 11:53AM
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I may have misled you. The two wires that I believe are coming from the stator under the flywheel are red and black and plug into a red and white wire. I have attached a picture of the wires. The red wire runs direcctly to a key cylinder that starts the tractor. The white wire runs to the light switch. There is also a black wire coming from under the flywheel at the front of the tractor that connects to a yellow wire that runs directly to the key cylinder. Hopefully, the picture makes sense. Is there a chance that the voltage regulator is located under the flywheel? Could this model not have a voltage regulator? Again, thanks for the assisstance.

    Bookmark   July 31, 2013 at 8:58PM
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You don't have a VR.
The "lump" in the RED wire coming from the stator is your charge diode.
It converts AC to DC. (3 AMP)
The other wire is your AC (5 AMP) lighting circuit.

The diode in the link is what many of us use for a replacement.
Polarity is important else you'll get negative DC out.

Here is a link that might be useful: Diode

    Bookmark   August 1, 2013 at 12:36AM
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OK, your system DOES NOT have a regulator.
Yours is the Dual Circuit system.
One circuit delivers untouched AC to the lights.
The second circuit is for battery charging.
It has a simple rectifying diode to change the AC output of the stator to DC for charging the battery, but the output is unregulated.
What sort of history does this tractor have? Has it always been in your hands or is it something you acquired "used"?
I ask because this type of unregulated charging system is usually just "barely able" to replace the drain of cranking a battery after about 30 minutes to an hour of run time.
Unless you "topped off" the battery with an external charger before you did your volt meter test (the test where you stated a reading of 16 VDC), this system really ought not be capable of raising the voltage to that value.....unless the engine is seriously over speeding.
You might do well to have the RPM checked.
How big a battery did you buy (cranking amps rating) to replace the exploded battery?

This post was edited by mownie on Thu, Aug 1, 13 at 0:43

    Bookmark   August 1, 2013 at 12:38AM
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Intrigued by the "battery exploded..." comment. In my experience with farm tractors, etc. that kind of failure is caused by a dead short on the battery. Even then, the contacts on the starting relay will usually fail prior to actual battery explosion. Usually, someone dropped something which "bridged" from + to ground.

Perhaps if the diode is shorted and there was pure AC on the battery would cause this? Maybe Mownie will weigh in on this one.

Good luck,


    Bookmark   August 1, 2013 at 9:21AM
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It is normal and expected for lead/acid batteries to generate hydrogen gas when they are charged. There should be some kind of vent to allow this excess gas to escape. If the vent was plugged, then internal pressure could accumulate, causing a sudden and catastrophic failure of the battery case. It would not be necessary for ignition to take place, this scenario would be more like an exploding balloon, that was overinflated. Since batteries are filled with a solution of water and sulfuric acid, it would be necessary to douse the contaminated wiring and hardware with baking soda solution, or something similar, to neutralize the acid. Clean cold water can be used to get rid of the baking soda solution.

    Bookmark   August 1, 2013 at 12:00PM
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When one of these diodes shorts, the main fuse for the electrical system will blow as soon as the key switch is turned................because..........the diode is no longer a has become nothing more than another connecting point in a circuit.
This means that it will conduct current in BOTH directions.
In normal operation with a good diode, the stator is only going to be able to make about 4 amps maximum (on this particular machine) to send out to the battery, and 4 amps will not overload the 15 or 20 amp rated fuse used in most tractors in this class.
But, on the other hand, the battery is capable of throwing at least 50 times greater number of amps back at the stator.......if it could just get past that darned diode preventing it from doing so.
And once one of these diodes becomes shorted, the main fuse will blow......and keep on blowing new fuses until the diode is replaced.

Diodes usually don't short out of their own volition while in service, though it can happen that way.
Diodes usually short out when they are asked to conduct an amp load that exceeds their rated capacity.
The surest way to exceed the ampacity of a diode in any automotive charging system is to subject the electrical system to a REVERSE POLARITY connecting jump start cables backward between the donor battery and the recipient battery...................or by installing a replacement battery carelessly and getting the cables connected backward.
It is relatively easy for a diode to resist the flow of current in the wrong direction (against its bias), and that goes nicely with the intent of allowing only one side of AC current from the alternator to pass through without damaging the diode......because the amps from the alternator are within the amp range the diode was made to handle.
When you commit the reverse polarity sin with an automotive charging system, you are effectively tasking the diode to carry a massive amp load back through the stator windings and ultimately back to ground.
And because the fuse is designed to protect the electrical system from damage due to current overload, the fuse pops.
Occasionally (but not very often), a diode might survive a reverse polarity event where the fuse popped before the diode melted together.
And to be realistic about the damage done when a reverse polarity event occurs.............the damage to the diode usually causes the diode to become OPEN, not SHORTED.

So, the prospect of a diode being shorted and passing continuous, pure, unrefined AC to the battery is impossible.
What is possible is for a diode to have its bias altered somewhat in that it might allow SOME AC to pass at intermittent times, and that would show up as a transient event if you connected a volt meter to the system with the meter set to AC scale instead of DC (as suggested by bill kapaun).
But even if the diode was allowing some transient AC to pass the diode, that WOULD NOT raise the voltage level of the electrical system.
Transient AC in a strictly DC venue will result in effective LOWERING of the battery state of charge because every DC pulse of current ADDS a bit of charge to the battery.
Every AC pulse of current ADDS a bit of charge, but then immediately SUBTRACTS a bit of charge.
You would think this might be an equal "give and take" resulting in "no change". But remember, every time a transient AC pulse enters the mix, it means one less DC pulse that would have charged the battery, or put another AC pulse negated some charge intended for the battery.

The voltage level of the electrical system in this instance is determined by the SIZE of the components (flywheel magnets) and their frequency (engine RPM).
Briggs has engineered this system to be a somewhat loose match between charging system output and the electrical needs of the machine.
It is anticipated (by Briggs) that the STARTER is the ONLY DC operated component on the machine and that it will only be operated briefly to start the engine. Once the engine is running, the alternator will SLOWLY (at a rate of not more than 4 amps) replace whatever charge was withdrawn from the battery during cranking the engine. Briggs calculated this to require an average time of 30 minutes to around one hour to bring the battery standing voltage back up.
The problem with an unregulated system is that it has no way of accounting for variables, such as engine RPM and/or long run times.
I think the engineers have done a remarkable job of figuring out the AVERAGES to enable such a simplistic version of charging system to meet nearly all needs in the applications where it is used.
But if you toss in some "unforeseens", like too many RPM, or run all day long (dependent of course on fuel tank capacity), or a battery that is too small for the application, a battery that is not filled to the correct level of electrolyte (yes, having a battery low on electrolyte is equal to having a battery with lesser capacity).
Any or all these things might contribute to having a system voltage rise higher than is optimum for the battery life.
It might be money well spent to install a "volt meter" (dash gauge) on this tractor and watch what happens with the charging system as the machine is used, from cranking at the start of a use cycle, to final shut down when the job is done.
If it seems that the the charging system is a bit too robust, then we may have to discuss some options.
but I would want to know for sure just how much "too robust" the system was before trying to bring it in line.

The Briggs service manual states that over charging can be caused by excessive vibration on the battery, caused by inadequate mounting brackets etc. (though I am at a loss to connect vibration of a battery to raising the charge level).
It also states about the battery being too small for the application or if the battery is damaged.

    Bookmark   August 1, 2013 at 12:18PM
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Wow Mowie, Bill, Others, you guys are outstanding. To answer/clarify a couple of the questions asked:

1. I acquired the mower secondhand about nine years ago. Since that time I have done routine maintenance including pulling the engine and replacing the shaft and oil seals.
2. The new battery is listed as having 365 CA at 32 degrees. I am not sure what the former battery was rated.
3. When the previous battery blew (literally ripped the case apart) I had just finished mowing (run time at full engine speed about 1 hour). I went to restart the mower to pull it into the garage and when I turned the key was when it exploded. (Just as information it was very hot and humid that day.) Prior to the event, the mower started easily from the battery (no need for external charging or jumping). Also, the battery cables did not have any corrosion on them.
4. I believe full engine speed (RPMs) have been constant since I acquired the mower. I will have to figure a way to test the engine speed to insure that it is not running too fast.

Having read the responses, it appears that the diode is not used to regulate current, only direction. Because the main fuse did not blow, and the battery is remaining charged, the diode should be ok and not need to be replaced? If you do suggest replacing the diode, can the fitting be taken apart and a new diode installed? (I am aware of the need to check for proper direction/current flow.) Also, there is a dash meter that indicates a positive charge on the battery while the tractor is in operation. Although there is a slight “bouncing” movement of the meter’s needle, it remains in the ok operation range. (I was not really sure how much I trusted the gauge and used my volt meter instead.) Again, the last time I finished mowing with the tractor my volt meter indicated between 15.8 to 16 volts at the battery with the tractor running fully throttle, which I believe is excessive and damaging for the battery. Again, thanks for all of your input.

    Bookmark   August 1, 2013 at 3:24PM
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A charging battery generates hydrogen gas.
Excessive charging generates a lot more.
Some battery explosions are simply a bad connection on the post that generates a spark igniting the gas.

The diode simply acts as a check valve.
It allows the positive pulse of the AC Sine wave to pass through, but blocks the negative half. The result is a pulsating DC current,

You still didn't specify if you had your meter set on DC or AC.
Not trying to be insulting, but many posters aren't even aware of the different ranges. The turn the dial until it reads "something".

A diode can also be mechanically damaged by being twisted around etc., causing it to short.
Use the OHMs function of your meter (you may have to try several scales) and check the diode in both directions.
It should conduct one direction, but not the other.

    Bookmark   August 1, 2013 at 4:19PM
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Bill, Thanks for the information. The meter was set for DC. Can you advise how the diode is removed for testing. I unplugged the connection immediately downstream of the diode. Does the black boot/covering where the diode is located pull out from the plastic connector to gain access to the diode? Thanks

    Bookmark   August 1, 2013 at 10:02PM
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If you unplug the connector (as you stated) you can use the terminal inside the connector for one test lead, but (and per Briggs recommendations) you may pierce the insulation of the wire upstream from the diode using a safety pin or a straight pin, or you can purchase electrical probes to do the same.
Once you have pierced the insulation, you put the second test lead of Ohmmeter onto the pin or probe to do the test of the diode.
I am not an advocate of poking holes in wires, but sometimes you have to.
I always seal the hole up with super glue or RTV smeared on the prick point if I can't slip a piece of heat shrinkable sealant tubing over the wire to cover the hole.
I have also used a product known as "liquid electrical tape" which is a brush in can vinyl plastic compound that you paint onto the wires and I like it too.

After learning your new battery has 365 CA I can say that your battery is not too small for the system.

Because the earliest history of this engine is not known, it is possible that somebody somewhere put an incorrect flywheel on the engine.
It might be a good idea to test the AC voltage output of the stator by disconnecting the stator connector as in the diode test above, except this time the engine will be running and you will set the voltmeter to AC scale.
With the engine running at governed RPM, touch one test lead to the pin or probe in the stator wire upstream from the diode and touch the other test lead to a clean engine ground.
This will reveal the maximum AC VOLTAGE that is being output by the stator and flywheel.
If your engine has the correct components to match the Dual Circuit system, the maximum voltage should be 14 volts (at 3,600 RPM).
If the voltage shows to be about 16 volts or so, I think your engine is doing too many RPM.
If the voltage shows to be around 28 volts, someone has done some mismatching of stator/flywheel in the past.
If the voltage shows to be 40 volts.................oh boy! Someone has really gone overboard with the wrong parts.
My gut feeling on this is "too many RPM" but I have been wrong before.

    Bookmark   August 2, 2013 at 12:58AM
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Aren't you supposed to have 14 VAC on the lighting side and 28 VAC on the diode side?
Since the diode blocks the negative part of the sine wave, only a 14 V "pulse" would pass through the diode (minus the "couple tenths" voltage drop of the diode)

OP, you might just check the lighting side first and see if that is the "normal" 14 VAC. IF it's 16ish (or more), that would tend to indicate problems.

This post was edited by bill_kapaun on Fri, Aug 2, 13 at 9:55

    Bookmark   August 2, 2013 at 9:50AM
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Very interesting topic going here. Wondering if the trees aren't getting in the way of the view of the forest though?

If there's 16VDC at the battery AND this isn't a regulated power supply which it isn't with just a diode that would indicate there's no current flow. Is there a meter on the tractor and does it indicate current going into the battery at the 16VDC reading? Is the OP taking a reading on the cable connector or on the "lug" that physically goes into the battery? Maybe a high resistance connection between the two which would indicate 16VDC but it's an "unloaded" voltage, e.g. no voltage drop to pull it down to 12+.

Still wondering how or why the original battery exploded.


    Bookmark   August 2, 2013 at 10:09AM
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You do see this from time to time and it seems to be taxing on batteries in the long run. IE the guy that gets a new battery every 2 years. Usually it gets picked up on a PM , you clean, service, check and watch the meter just tick upward. 32, 33, 3500 RPMS not seeming to change the factor by any great margin. Not referring to the OP specifically here, I see enough times where a new battery is put on -sometimes not using a wrench at all by the looseness of the terminals as it starts to create other issues- which as pointed out above a spark and a battery doing its thing don't mix.

    Bookmark   August 2, 2013 at 11:00AM
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The engine in this thread is fitted with the Dual Circuit charging system.
According to what I can glean from the Briggs Engine/Alternator Replacement Guide, the stator in the Dual Circuit system is (should be) able to output 14 volts AC.
Whether you are checking the dedicated AC terminal, or the DC lead (upstream from the diode)........the voltage valve ought to be the same, whether AC or DC.
Rectifying 28 VAC does not yield 14 VDC.
Rectifying 28 VAC yields 28 VDC, but in half sine pulses.
Rectifying 14 VAC yields 14 VAC in half sine pulses.

In a rectified, but unregulated system, observed battery/system voltage will climb to reach the exact (or near) voltage value of the charging source ONLY when the battery has ABSORBED its optimum charge. If the battery is still lacking some charge, the observed voltage of the battery/system will be LESS than the final voltage the charging system is capable of.
ONLY after the battery is completely saturated (charged up) to its optimum point will you be able to observe a voltage level that matches the design parameters of the charging system.
So, if you are observing a voltmeter connected to the battery/system of a Dual Circuit system, and the battery is not TOTALLY CHARGED will not see 14 VDC.
Remember, this is one of the minimalist DC charging systems 2-4 amps max, so if the battery is not yet fully saturated, the VOLTAGE observed will be less than the design maximum output voltage of the charging system.
As the battery nears saturation, the observed voltage value will raise.
If the battery reaches the maximum state of charge on an unregulated charging system, the battery/system voltage will continue to climb, until it matches the ACTUAL output voltage that the charging system is making at that time.
And any time you exceed the optimum voltage of a battery, you are not actually charging the battery any longer, you are simply carrying on electrolysis, and boiling the juice out of the battery.

If this engine in this thread is outputting more than 14 volts (AC or DC), I believe it will be due to excess RPM.

    Bookmark   August 2, 2013 at 1:04PM
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I was looking at some Craftsman schematics with the same system and they showed 28 VAC on the alternator side of the diode.
Of course we know about "some" Craftsman schematics........

    Bookmark   August 2, 2013 at 1:16PM
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In general, lead/acid storage batteries will last longer if they are securely clamped down. They are adversely affected by excess vibration. Since the owner has spent some time getting this lawn tractor maintained and operational, he might want to modify the wiring with a voltage regulator, to keep the battery from overcharging in the future. It would be easy to install a voltmeter at the same time, to keep an eye on what the charging system is doing.

    Bookmark   August 2, 2013 at 10:15PM
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".. he might want to modify the wiring with a voltage regulator, to keep the battery from overcharging in the future."

How much do you think this idea would cost?
New flywheel >$200+
Stator $40-60
Regulator $30

    Bookmark   August 2, 2013 at 11:09PM
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It would be possible to purchase n=10 5 amp diodes, pn 50SQ080, at 85 cents each, and build up a system for keeping the battery charged, without overcharging. Four diodes are used to make a full wave bridge, & this assembly will lower the stator AC voltage by one volt. To further reduce the DC voltage, at the battery (+) terminal, additional diodes are wired in series, forward bias, as needed. Each diode reduces the voltage by 0.52V. If the DC voltage at the bridge output measures 15V, then 5 diodes in series will lower the voltage at the battery terminal by 5X0.52=2.60V. That would result in 12.4 volts at the (+) battery terminal, and should eliminate overcharging. This is a straightforward circuit that is easy to build and troubleshoot. The starting point is to find out the actual AC voltage that the stator is producing, over a range of operating RPM.

    Bookmark   August 3, 2013 at 4:51PM
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A ONE input/output Bridge rectifier?

    Bookmark   August 3, 2013 at 5:00PM
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Original poster back with an update. I was able to measure voltage on the unit and this is what I found. On DC mode at the battery with engine not running 12.8 volts. Testing upstream of the diode with the meter on AC 16.6 volts (at full throttle). Testing at the battery connection in DC mode about 16.4 volts (at full throttle). By lowering the speed of the mower I was able to drop the voltage as measured at the battery to around 14 volts with still plenty of power to run the tractor and mow the yard. I am thinking that the engine speed on the tractor may be excessive as suggested. (I do not have an RPM or tach to determine engine speed.) Can you tell me if this engine has a mechanical governor that can be adjusted or is it adjusted simply through the throttle cable? Thanks for all of your input.

    Bookmark   August 5, 2013 at 9:18PM
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" Testing upstream of the diode with the meter on AC 16.6 volts (at full throttle)"
Is this with the battery connected?
IF so, the diode is shorted because you shouldn't be getting AC, just DC.
IF the battery is disconnected, you may getting a slight "bleed through" of the AC.
Meters, especially the newer ones, draw so little current that you can sometimes get a false reading.

    Bookmark   August 5, 2013 at 9:31PM
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Testing as indicated in the image below will disclose the maximum, AC voltage output of the CHARGING leg of the stator.
AC is tested by putting on test meter lead to the wire upstream of the diode with the white connector halves separated. You will have to prick the wire insulation as described earlier in this thread.
If you want to check for transient AC, put your test meter on AC scale and touch one lead to the battery positive post and the other lead to the battery negative post, or any good ground point.

    Bookmark   August 6, 2013 at 1:07AM
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