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Charging SLA Batteries in Series & Parallel

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Mike Hamilton-Macy
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Posted on Tuesday, September 20, 2005 - 9:50 pm:    Edit Post    Move Post (Moderator/Admin Only)
Hi Paul,team Wind Power here.
Pressure uses 4 x 12v/7a sla batteries (alarm backup type), 2 pairs wired in series then parralelled to give 24v/14a. We are looking for something better than a car charger that we currently use! Is there one unit that could charge all 4 together or would we need 2 or 4 chargers?
What do you suggest? (bearing in mind cost).
Thanks for your time, Mike.
Admin (Admin)
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Username: Admin

Post Number: 77
Registered: 10-2004
Posted on Thursday, September 22, 2005 - 6:54 pm:    Edit Post    Move Post (Moderator/Admin Only)
Hi Mike

An interesting question and one in which I have some views to which others may disagree:

Discharging batteries in series is fine of course.

Charging batteries in series has differing views. My view is that it is fine (although not ideal) as long as the batteries are in identical condition. This means always charging and discharging the batteries as a set - don't mix them up with other batteries.

Discharging in parallel is a risk, we certainly use Ni-cds in parallel but beware that a failure in one battery can cause a catastrophic failure and risk of fire etc. What can happen if a cell in a battery fails is that the good battery in parallel with it can discharge into the failed battery / cell. Good practice is to only connect batteries in parallel when they have equal states of charge, never leave the batteries in parallel after use.

Charging batteries in parallel is not recommended at all, the batteries can charge at different rates potentially under-charging one and over-charging the other.

So, in answer to your question:

Ideally use four 12V chargers, a minimum of two 24V chargers (2 x 12V in series).

The size (output) of the charger should be between 0.2 and 0.25C (battery Ah capacity). As you are using 7Ah, the charger output should not be greater than 1.75A. Note some makes of battery do allow higher charging rates but these are less common.

Technobots currently do not have any suitable chargers although we are about to source some from China. The smallest 24V charger we hold is 10A output - too big!! We have a greater selection of 12V chargers but they are typically 500-700mA which means charging time is greater or are too large 2.7A and above which could damage the battery.

I could suggest using larger capacity batteries, say 12 or 17Ah which are about £20 each and our 12V 2.7A 3-stage chargers at about £27 each.

Hopefully that has helped a bit.

Paul