How do I design a new battery system using two old but large AGM batteries in series?
I realize a load test must be done to determine capacity differences.
AGM batteries are also sensitive to overcharging, and voltage is temperature sensitive.
What are the key critical design specs to define? I can follow this to implement a reliable design.
I have a floor scrubber that has four 6V flooded lead acid Trojan T-125 batteries. Problem is after using it to scrub the floor, it takes about 4 hours to recharge using the onboard 21A max charger. Using a more powerful charger on them is not recommended as the batteries will heat up excessively and likely shorten their life. I have a candidate solution though and wanted to know if it will work and what problems may arise. At home I have 2 large (but identical) Odyssey PC2250 12V AGM batteries that are quite old (at least 10 years) but have a rated capacity of 126Ah. They do not hold identical charges and one has slightly higher actual capacity than the other but they have not been in service for many years but I maintained them. They usually hold 12.7V OCV (Open Circuit Voltage) which tells me they are still in decent condition.
So here is my plan and I would like to know if this will work or if I am headed down the wrong path. I will do a load test on each of these batteries using my 12V power inverter and 500 watts of incandescent lights as the actual load. I will simply measure the duration of the test. The inverter will beep when the load voltage drops to about 10.5V. At that point I will shut off the inverter and put the battery back on the charger for a full charge. I will do this same test for each battery solo and probably twice for each battery to confirm the results. So assuming they are both similar Ah capacity (let's say the test lasts 1 hour each which would be about 42Ah each since 500W = about 42A at 12V), would it be ok to put these back into service but with a new task as floor scrubber batteries? To get a quicker charge, I would use an external charger with 55A charge capability which will far exceed the onboard chargers maximum 21A rate. I expect the actual charge time to go from about 4 hours on the old system to maybe 1.5 hours on the new system.