Can Different Capacity Batteries Be Charged Together

Question:

Will different amp batteries, of the same voltage, hooked up in parallel charge safely? For example I am hooking up 100 amp 12 V battery in parallel with another 270 amp battery. My charger is rated for 500 amps. I am wondering if it will overcharge the low amp battery, if it will not fully charge the higher amp battery. Thanks for any insight

asked by: Matt J

Expert Reply:

Batteries should always be charged individually.

If you had two identical batteries but one is new and the other is a year old and has been through many charge/depletion cycles, the new one will pull in more of the charging power because it is in better shape. Batteries age and become less able to accept a charge effectively. Batteries connected together can result in one slowing the rate of charge on the other.

Batteries with very different Amp-Hour ratings, like a 100-Ah and a 270-Ah connected together, will not charge correctly either.

Most quality chargers have a safety feature to prevent over-charging. For example, the NOCO Genius 3-Bank Charger # NOC94FR includes this feature along with many others. Multiple batteries connected to the charger at once will lose this feature since the charger sees only the total load and will push charging current until it senses full charge on the total load.

expert reply by: Adam R