Using a battery balancer

OntheWay
OntheWay Solar Expert Posts: 36 ✭✭
Hi, I am using mpp solar 24V inverter charger with two 12V 110ah lead acid batteries, system charges only from grid and used as energy backup in lack of grid electricity.

While my batteries are rarely used,  it's 5-6 years already, and cannot hold load, I will replace them. I read somewhere, lead acid batteries quickly loosing balance, therefore a balancer is required. What's your opinion about it? Since I don't know how they work, my concern is if it interferes with charger and misleads the charging algorithm, found on inverter.
Really appreciate any opinions.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,404 admin
    Generally, Flooded Cell Lead Acid batteries do not need/use a balancer... Equalizing charge is a mild over charging of the battery bank. The fully charged cells are overcharged (and "gas") and let around 2.5 to 5% rate of charge current through--And this current charges the "weak cells" and brings them back to full charge.

    There are two suggested times to EQ a FLA battery bank. One is once per month. And the other is to EQ when the cells are out of balance when measured by their specific gravity. When the SG from low to high cells is greater than ~0.015 to 0.030 SG units, then EQ is done.

    EQ is somewhat hard on a FLA cells, so you only do it when needed and for a limited amount of time.

    Sealed lead acid batteries, like AGM cells typically do not get out of balance very much. And EQ for AGM batteries is just holding the absorb setpoint for 8-24 hours once every ~6 months. You do not want AGM batteries to generate much gas or they overheat and can vent hydrogen+oxygen, which can reduce their life by a lot (you cannot add water to AGM/Sealed batteries, and excessive gassing overheats/wears out the catalyst). AGM cells do not let much current through when EQing... So cells with different state of charges (such as a 50% charged battery in series with a 100% charged battery) is not a good thing.

    Battery balancers are usually used with various Lithium Ion type batteries were controlled over charging (high voltage, and low voltage cells) are quickly damaged/ruined and the BMS (battery management system) is used to prevent cells from diverging state of charge and over/under voltaging cells.

    For Lithium Ion batteries, a BMS system need access to each ~3.4 volt cell. For typical (modern) lead acid batteries, you do not have access to each cell (no exposed bus bars). So a lead Acid BMS would be wired up for 6 or 12 volt batteries--And you would not have a per cell BMS--But just a per battery (group of cells) BMS--And you would not have the advantages of per cell BMS. If you had a larger battery bank (large AH capacity), you could get 2 volt cells and use a 2 volt per cell BMS and get the full advantages (and the "mess" of wiring up 12 sets of BMS wires--One wire for each 2 volt cell, 12 cells total for a 24 volt volt battery).

    Depending on FLA battery "quality"--Lower cost batteries typically have a 3-5 year life. For applications like yours, 6 volt @ ~200 AH batteries are a good price/performance value. They are available in FLA or AGM formats (AGM being more expensive).

    For lead acid batteries---Under charging, over discharging, and running hot tends to be major factors in short battery life. Checking battery voltage and specific gravity per cell (if FLA) gives you insight to battery charging. And keeping them cool (cool room or basement, vs a hot/sunny room) helps. The old engineering rule of thumb... For every 10C increase in temperature over 25C, battery life is cut by 1/2x (18F over 75F).... Our friends in the great white north can get very good battery life because of the cold temperatures their batteries operate in.

    And for lead acid batteries, there are those designed for cycle service (deep cycle storage batteries). And those designed for "float service" (computer UPS systems and such). Deep cycle batteries tend not like to sit at float charge. And float service batteries tend not to like deep cycling (may need to replaced after a few deep cycles--Aka power failures).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • OntheWay
    OntheWay Solar Expert Posts: 36 ✭✭
    edited February 2020 #3
    Wow, thank you very much for all this info, still digesting.. my batteries (two of them in total) are sealed lead acid, non-AGM. My balancing idea was between these two, and not within cells of a single battery(sealed already). But today later I read somewhere Mppt chargers don't like the battery balancers, may be ruining the charging algo..
  • BB.
    BB. Super Moderators, Administrators Posts: 33,404 admin
    Measure the voltage across each battery. The voltages should be nearly the same.

    In this case, I would guess nearly the same means less than 0.180 to 0.090 volts or less is good (12 volts battery assuming around 0.015 to 0.030 sg units difference between cells).

    You can use a 12 volts battery charger to bring up a weak battery. Or clip on a 12 volts brake lamp (roughly 1 amp) to slightly discharge the high battery.

    Or hold around 28.8 VDC for 24 hours on your battery bank (it the voltage recommended by your battery mfg.).

    If the batteries are the same voltage testing, but one drops a bunch under load, then it is probably time to replace the bank.

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • OntheWay
    OntheWay Solar Expert Posts: 36 ✭✭
    Thanks, I have a Bosch C7 charger. I will try regeneration charge, after letting the bank drain down by load.