24v to 36v, one of three batteries is different.

[webertime]

I have a 24v set up currently. 2x 750cca 109ah. I have the opportunity to jump up to a 36v (barely used 101lb for free). I have a spare DC battery that is still good, but its 730cca and 100ah. What would happen if I added it to the existing pair I already have ?

[Ann-Marie]

If one of the batteries runs down while the others still have substantial charge they will force current to flow through the dead one and kill it rapidly. Normally all three should be balanced for capacity so they discharge uniformly and don't fight each other. It will work fine but you should keep a close eye (voltmeter) on the weak one and stop using it if it gets too low.

One way to extend running time is to use the Trollbridge36. Every time you run the main engine it will put the 3 trolling batteries in parallel and charge them from the alternator. While they are in parallel, current will flow from the "good" batteries to the weaker one and equalize the charge. The additional charge from the motor will not only extend running time but it will also extend battery life by reducing the depth of discharge.

[Lou r Pitcher]

Two 109AH batteries in series with a 100AH battery should if all flooded lead acid healthy batteres discharge somewhat similarly as the capacities of all 3 are very close....there's less than a 10% difference.

Always best to monitor all battery voltages on the water and recharge before the weakest battery becomes overcharged....true even if all three were the same model and same age. Especially important to monitor battery voltages if one of a series ran battery has less capacity than the others.

Anytime a charge sharing device parallels batteries that are at different states of charge, the stronger will discharge its chemical provided current into the weaker as correctly mentioned above.

Not stated was that the action of discharging one battery into another always reduces the maximum possible life available from the stronger battery though they will equalize. There simply are only so many potential discharge/charge cycles available from any chemical battery.

Whether additional charge is available from the alternator to significantly benefit the now partly depleted stronger battery having just lost some of its charge to the weaker paralleled battery is dependent upon both whether the engine is ran long enough period of time and whether engine is maintained at a sufficient RPM. That depends on the boater and varies.