Rotating Batteries

720Deere

I know it sounds like a weird concept, but after recently noticing the two batteries in the middle of my pack using more water than the others, I carefully monitored them all during charging today. The two suspect batteries are charging at least 0.1v less than the next 2 in line and the two that are connected to the controller are 0.1v higher than middle batteries. So basically I have a 0.2v variance between the left and right batteries during charging. All but the two suspect batteries have the same at rest voltage with the oddballs being 0.1v lower.

Is it normal for the batteries connected directly to the charger to charge at a higher rate and the batteries furthest from the charger to be at a lesser voltage? If this is the case, would it make sense to swap the right and the left batteries periodically to help keep things even?

BGW

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Revolution

I am curious to see the responses on this. My old pack (4-12volt) the outer batteries used more water than the inner two did.

How old is your pack? Could it be the batteries just getting old? Maybe Charge the weaker ones separately?

Great white north

........ Interesting , look forward to discussion

kellybell

That small a voltage dif could be the DVM error.

kernal

This was discussed briefly some time last year and brought up the subject of "bank charging". I can't remember who started the discussion or the right key words to find the threads. There was a similar thread in the club car section which is sort of the same in that it discusses a way to balance a mixture of batteries of different ages in the same pack. Look here: http://www.buggiesgonewild.com/elect...v-battery.html

720Deere

Thanks for the link Kernal! Interesting reading for sure, but it still doesn't address whether or not position in the pack has any affect on the batteries. I just find it odd that the batteries at the the ends of the pack always carry a higher voltage and the batteries at the middle of the pack carry lower voltage. For almost a year the lights were connected to 2 batteries, but only one of those was one of the lower voltage pair.

radioman

The charging current in batteries connected in series is the same on each battery. The same current flows in every point in the battery bank assuming there are no 12 volt taps powering anything while charging or any leakage caused by dirty battery top surfaces. Tapping batteries to power 12 volt devices is asking for early failure.

yurtle

In other words, rotating batteries will have no effect, and is not necessary.

JohnnieB

The batteries are charged in series, so an identical number of amps flows through all of them for an identical length of time, however no two batteries are exactly the same storage capacity, so the On-Charge voltages will not be exactly the same. The closer the batteries are matched, the closer the On-Charge voltages will be.

On the other hand, The At-Rest voltage is tied to the SoC (State of Charge) and will be the same when the battery is fully charged, regardless of its storage capacity, if the batteries were all filled with the same acid concentration at the factory and none has spilled or otherwise lost. (Only water is lost through gassing)

Technically, a battery isn't fully charged until the SG (Specific Gravity) of the electrolyte ceases to increase while there is a charging current passing through the cells. Since the On-Charge voltage is tied to the SG of the electrolyte, it can be inferred that the On-Charge voltage also ceases to rise when the battery is fully charged.

Battery chargers that monitor the change in voltage over time (dV/dT) and terminate the charge cycle when the On-Charge voltage stops rising, or slows to less than a predetermined rate of change, tend to charge all batteries to a higher SoC and more equally than chargers the terminate the charge cycle when the voltage of the entire series string reaches a fixed predetermined On-Charge voltage.

Slight differences in On-Charge voltages don't actually mean too much since it is the number of Amps over the length of time that actually charges the battery. However, a deviation more than 0.25VPC (Volts per Cell), which is 0.75V for a 6V battery, from the average On-Charge VPC for all batteries in the pack, may indicate a problem.

To get an accurate On-Charge voltage reading, measure from Lead post to Lead post rather than from the studs or cable lugs. That eliminates the resistance in the cables and connections.

On the other hand, the At-rest voltage tells you how fully the batteries are being charged. They should all be within 0.1V of the pack average. If they aren't, initiate a new charge cycle after the charger shuts off automatically. (aka equalization charge). This may have to be done several times to get the all within 0.1V and then about once every moth or so to keep them that way.

Also, giving the battery(ies) with the low At-Rest voltage a supplemental charge with a 6V charger (or charged in pairs with a 12V charger) will bring them in line with the others.

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I'm mot sure why the center-front and center-rear batteries use more water, but mine do also.

yurtle

I've never noticed this, but wasn't really paying attention either. Middle batteries are protected from the environment more that the outer batteries. Only one side is exposed, while the other batteries have two sides exposed. Could be temperature related during charging, storage, and use?

Next time I fill mine, I'll see if I notice a difference within the normal pack and the two extras, which are exposed on all four sides. Maybe gassing occurs more on the "protected" batteries than on the ones with more exposure?