Batteries Swelling Up, Need Advice

i have a friend of mine who we installed a huge system in his 94 chevy tahoe, at least i think its a 94. the system consists of two 18" MT sub woofers, two autotek mean machine 4000D's, and 7 kinetik HC3400 batteries. the batteries are being charged by two 250 amp high output alternators that we had built by a local alternator shop. the major problem is that the batteries have all swollen up and look like they are ready to burst. at first it was only one of the batteries that was swelling, which just happened to be an older battery than the others so we just chalked it up to the problem being that it was an bad battery. we took the battery out of the system and a few weeks later now all the batteries are swelling. the entire system is ran with high quality 0 gauge wiring. all grounds have been upgraded and are bolted to the frame directly with the coating sanded down to bare metal and then re coated with undercoating once the connection was made. all positives and all negatives on the batteries are hooked together so i guess that would be parallel wiring. the batteries are grounded to the frame at different points along the battery bank, which i think could be a potential issue but im not sure. i am thinking that the batteries are just being very over charged and its causing them to swell up from internal heat. also the alternators are hooked up to an external regulator which is being fed only from the battery bank and not the main vehicle battery. the vehicle charging system and the audio charging system are completely separated . if anyone can shed some light on these issues i would greatly appreciate it because these are expensive batteries and this cant keep happening.

What is the output voltage of your alternator? I have seen this before, but there was a problem with the internal voltage regulator. The alt was charging at a little over 16 volts.

the voltage of the alternators is controlled by an adjustable regulator. i have it set very low and it will sometimes jump to about 17 volts and then level off after a few seconds. its supposed to do that when it sees a a drop on the field wire voltage which is hooked directly to the battery bank but it is isolated by a regular relay that is activated on ignition. this is the way i was told to wire this setup by ohio generator tech support and others. the regulator was bought from ohio generator and the alternators are custom. the problem is that its not my car so i cant be with it 24/7 to monitor what it does. we planned to wire in some voltage displays to make things easier but money got tight so those had to wait.

It's probably thermal runaway. It can happen in any dry battery - ie, a wet cell with fluid boild off, or and AGM batery. That swelling is a precursor to fire or explosion. It can be caused by a collapsed monoblock being fed by other parallel batteries (which then fails or collapses the parallel good batteries). Or the alternator which is overcharging - eg, charging at normal voltages which is too high for faulty batteries, or itself outputting an over-voltage (eg, >14.4V). Replace the batteries, fit appropriate isolators, ensure the alternator senses the battery voltage (ie, an internal regulator with a Sense line to the battery +12V terminal) and does not exceed 14.4V output etc.

what do you mean by a collapsed monoblock?

A lead-acid cell is 2V. A 12V battery is 6 cells in series (6 x 2V = 12V). A monoblock is a "single block" or housing of cells. EG - a typical 12V car battery is a 12V monoblock.    - a typical truck 24V battery is TWO 12V monoblocks (in series). A collapsed monoblock is one with at least one collapsed cell - ie, a cell has shorted or partially failed. Instead of being a 12V battery or monoblock it'll be a 10V or 10.5V etc monoblock, hence normal 12V charging overcharges the block (due to over-voltage). "Battery" strictly speaking refers to the "total" battery. EG - a group of monoblocks connected together - whether in series or parallel - is still a battery. So ten batteries equals one battery. Just as six 2 volt batteries (ie, cells) in 6 monoblocks or two 6V mononlocks or a 12V monoblock is a battery. Confusing? No - just cautious when reading or referring to batteries.    AGMs swell or distort due to heat. The heat is from overcharging meaning too high a voltage and (hence) too high a current. Unlike wet cells, AGMs have no water to boil off to keep them cool. (The same can apply to gel cells.) And the lower internal (or cell) resistance of an AGM means a higher current than wets wrt to accepting charge and supplying charge (current). An AGM develops a faulty cell. It may collapse to being a 1.5V cell instead of 2V. The other 5 cells are hence being overcharged to hence they start to degrade. Charging current increases due to the AGM's collapsing voltage which means more heat and a faster rate of collapse. That's the thermal-runaway part. The charge current can be supplied by another parallel AGM or battery with or without an alternator or charger - the result is the same... The alternator supplies up to 14.4V for accelerated destruction until it is overloaded. When the system voltage drops to that of the paralleled good battery(s), the parallel batteries charge the faulty battery. As those batteries are being undercharged or are left discharged they too start to collapse. Within days or weeks you can have all useless monoblocks and a burnt out alternator, tho that can be hard to ascertain from the ashes of the possible fire or shrapnel of the possible explosion. I love it! It's a perfect example that highlights how correct people are when they say it is perfectly ok to parallel batteries (without using an isolator).

so are you saying that i should have used an isolator relay between each battery to keep them from feeding off of each other if one battery decides to die?

Yes.

The batteries were being overcharged.

i am an idiot wrote: The batteries were being overcharged.

This.  You don't need to isolate batteries in a bank that are wired in parallel unless you want to charge them separately or something.

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