'00 Suzuki battery ballooned


Staff member
It is a GSX-R750 with AGM (absorbent glass mat) battery. The attached pictures show how it has bulged out on all four sides and the top. The charge was 12.9V when I first tested it, but that dropped quickly under load (10 ohm => 1.29V, 220 ohm => 8V). I'm thinking this was severely over-charged, and am grateful that it didn't leak or even explode. What do you guys think? I think traditional "wet" batteries fine for most applications, see no need to waste money on fancy types, but this is and example of how AGM may have been a real blessing!

The back-story: This battery has been in service for a couple of years. About a month ago the charge was down enough for the bike to not crank. I determined that the alternator was OK, but the regulator was bad, so replaced the regulator and all was well until today. I'll put another battery in tomorrow and check things again, but I'm pretty sure I'll find the charging voltage is way high. This is a rather crude charging system, the alt has flying-magnets as a "fixed" field, so out put is always maximum. It could rise quite high as engine rpm increases (12K+ rpm), but the regulator shorts the alt (full current) output "as required" to keep voltage down to desired 12-14V range. Yep, many times the regulator is dumping 300W or more as heat. If anybody is curious, I have a copy of the FSM, thanks to nickb2, with schematics of this poor design. I'll contact the(aftermarket) regulator people tomorrow after I confirm it only lasted about a month.



Wrench. I help when I can
Wow Bill. Nice pics.

I probably told this story here before. But when a car battery exploded next to me from my knocking out some skid-do clutch springs. I was lucky to still have eyes. I was literally pulling off my clothes with my bare hands.

Since I am not very affluent in battery tech, I just see that battery as being sediment-ed so much, the lead just lay down and no charge will ensue. When that happens, they are literally a walking time bomb. You got lucky Bill. You caught it just in time. In your case. a AGM battery in principle should not sulfate.

I was charging a truck battery, think it was a ranger. So while I was banging the clutch springs out in the vice, the work bench just moved enough to spark. Clamp of that sun charger where nowhere to be found for weeks.

It was careless of me to have a charging battery which I knew was sulfa-ted that close to me.

I see the battery vented in three spots between the + and - on your first picture. Yeah. kinda of makes a guy say HMMM.

You need to really overcharge a AGM for them to explode.



Wrench. I help when I can
But they do. Never underestimate the power of hydrogen.

AGM or not, any battery under high load will do that.

I am just happy you did not get hurt.


Staff member
Well, I idled it until hot then revved it way up a few times and held it at a "fast idle" (6-8K rpm) for a few minutes; voltage held at about 14V. With nothing clearly wrong I told my son (his bike) to just use it some more with the new battery, let's see what happens. I'm not thrilled with that plan, this battery sits right behind the gas tank and right under his nuts. If it does anything dramatic while riding it could be quite unhandy, especially if he is "lane-splitting" in commute traffic.
That's about all you can do except convince him not to ride anymore.

I know that doesn't work. Been there done that.


Staff member
Well, no joy here. He noticed yesterday that the (new) battery was smelling "burned". When I got to it, 90 minutes after it had been shut-down, the battery was still too hot to hold my fingers on. Voltage was about 12.8, and rose to about 13.8 when the engine was running. Still nothing obvious, except the over-heating battery!

So, I'm struggling to not only get a handle on what the problem really is, but what to do for a temporary fix, as he wants to use the bike again for commuting. Ideally, I would like to log the system voltage so that I could prove to the VR manufacturer (and myself) that the VR is flaky. I don't want to just throw a VR at it. But making, or even buying, a logger in just the next day or two doesn't seem possible. Likewise, getting a new VR that quick isn't possible; even going to a dealer.

So, I'm kind of thinking of re-purposing an old relay-type auto VR to simply disconnect the bike alt/VR if-and-when voltage goes high, as a temporary measure.

Thoughts, suggestions?


Wrench. I help when I can
So, I'm kind of thinking of re-purposing an old relay-type auto VR to simply disconnect the bike alt/VR if-and-when voltage goes high, as a temporary measure.
Makes sense. As long as the sense feed is being monitored, should be a good thing. For a bike like this, probably a mechanical power switch is the best option for your problem.

I think you are on to something. Just try to convince your son to not drive this bike until it's fixed. I would hate for him to get burned in his jolly bag.

He is literally sitting on a bomb. ;)


Wrench. I help when I can
You are after all just trying to regulate the voltage, so amps are not a problem and low volt wiring will do in a pinch.

Maybe even a cut off switch like this would serve you right.

Screenshot (737).png


Wrench. I help when I can
On my old yamaha seca that my dad gave me, same problem. I had to trip the bike and it still shorted and all the wiring melted and what a mess to fix.

The seat also got burned, I was 19 yrs old at this point, not very good at what I do now. To boot, the gas tank got pierced.

I liked that 750 but it was a danger on wheels to say the least. Never actually rode that bike. Sucks cuz the engine and carbs were all fine tuned. Dad swapped it for a virago, and he never went back. Three viragos later, still trying to convince him to buy a 883 sportser. Probably wrote that wrong.

Dreams, and a 1340 would be nice, but he is short, so a low rider is what he needs.

At 5f 6or7 inch tall, I am the tallest person aside of my uncle who is 5f 8in.

Anyway, I am blabbering now, get a switch on the field, all should be kosher. You are way smarter than me. Re purposing that relay might be a very viable option. So again, I am just repeating myself.

Good plan. So long as the VR stays at 13.5v or so, should be fine.
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Staff member
Nickb2, the field on this alt is not controllable; it is "flying magnets", not a wound electromagnet. The VR controls output by shorting it to ground, dumping full alt output (about 360 W) in the VR components and alt windings much of the time. The VR normally gets very hot, so maybe this new one is a cheapie that can't take it. Of course, the OEM VR was acting up for a long time, too. It's a crappy system, I just have to figure out the best way to deal with it considering that nobody stocks parts, parts tend to be pricey, and my son really needs to keep it in service for commuting to work. Down-time is painful. Too bad 1KW 14V zener diodes aren't cheap and readily available, I could put one across the battery, redundant to what the VR is supposed to do (with SCRs).


Wrench. I help when I can
I probably read wrong your last post, but I think you may have typed kilowatt instead of kilovolts.

So if I understand correctly. Flying magnet in shunt mode all the time. Isn`t there supposed to be a resistor in circuit to trip voltage spike?

Not my writing, this is all I could find, this of for BMW but should apply to you.

How The Regulator Works
When the voltage produced by the generator is low, the current flows through the field windings to
ground. This works to increase the voltage produced.
When the voltage increases above 7 to 7.5 Volts, the current passes through a resistor which prevents the
voltage from rising so fast.
If the voltage continues to rise, the voltage coil shunts the field winding (both ends connected together),
which prevents it from flowing current. The voltage produced by the generator then drops to zero.
The current coil of the regulator measures how much current is flowing from the generator to the battery
and all other electrical components. It works to drop the voltage if the current rises too much.
The regulator also has a circuit breaker - this is normally open until the generator voltage rises above 6.5
Volts, when it closes and opens a path from the generator to all the consumers. When the voltage drops,
the current coil is wound so that the battery drains into the generator, and the current coil presses the
circuit breaker open.


Wrench. I help when I can
So back to basics for me,

again not my writing.

Replacing the Regulator
The regulator is a big compromise in order to get good results in normal conditions.
The biggest problem is the function of the current coil. It protects the generator from currents that are too
high, but as soon as there is a current flowing it is acting to regulate the voltage down. So with a rising
current, the voltage drops gradually. This produces a compromise - the voltage is too high when the lights
etc are turned off, boiling the battery, and it is too low when everything is switched on, causing the battery
to be drained.
The regulator is also sensitive to temperatures. The higher the temperature, the higher the generator
voltage. The regulator is mounted in quite a hot place, so at least the battery shouldn't starve - but it might
cook in summer.
The best compromise is to use a 14Ah battery, and have the regulator adjusted at the low end of the
voltages. Keep an eye on the battery, and if it is draining, give it extra load, such as by using the
headlights all the time, for example.
The circuit breaker isn't great either. It is switching currents of between 2.5 to 9 Amps, which isn't healthy
for a mechanical switch. This is a major issue when riding for long times at slow engine speeds.
The solution is to use an electronic regulator - this does away with the problems of mechanical switch
contacts, and most electronic regulators have a constant voltage over the whole current range.
An electronic regulator also has a diode to replace the circuit breaker. This acts as a one-way valve and
only lets current flow in a single direction.
Not all electronic regulators are suitable - they must have over-current protection as well, so that when the
current is too high for the generator, they switch off the field current.
But they are still temperature sensitive - they need to be kept below 80°C, so should be mounted outside


Wrench. I help when I can
I see that a diode will help the generator from dumping into the battery and boiling it.

Here is an interesting read, might work for your sons cycle.


Seems simple enough to put together in a pinch. Will probably outlast the bike.

What is the cut in voltage at?? Should be something like 6-7 volts. Under 6, your just driving on battery juice. Not long for the road.

Does his bike have a voltage meter or just an idiot light?

So on a funny note. Good to know the charging system works