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Why do my batteries suddenly discharge?

If I connect three batteries in series, will the center battery discharge differently?

  • I have three 12 volt batteries connected in series to provide 36 volts to my electric bicycle. Recently the battery in the center began discharging more rapidly that the two on the ends of the circuit. Is there any reason why the battery in the center would charge or discharge differently or is this just coincidence?

  • Answer:

    All three batteries charge from the same charger at the same current rate and discharge at the same rate through the 36 Volt load. But there is a possibility that the center battery has a 12 Volt load (accessories) connected to it in addition to it`s main function of helping the other two batteries in furnishing the current to the 36 Volt load. This would cause it to discharge quicker than the other two. However more than likely the center battery has just aged faster than the other two batteries. Even if all the batteries are identical and are the same age it is possible that the center batteries` internal internal resistance has become higher when fully charged than that of the other two batteries internal resistance when they are fully charged.. If so it`s terminal Voltage under load will be measurably lower than that of the other two. In this case the center battery needs replacing. When the useful time between charging becomes so short that it is a nuisance then you might as well replace all three batteries because the other two batteries will not last very much longer. As a battery ages it`s internal resistance becomes higher due to portions of it`s plates becoming inactive from corrosion, warping , etc. It`s Amp-hour capacity between charges diminishes with age. In applications such as the one you described all the batteries never age at the same rate. Therefore you can not expect all three batteries to become defective at the same age.

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All things being equal (i.e. identical batteries), they should all discharge equally because the current in a series circuit is exactly the same anywhere. I'd say most likely the center batter discharges faster is because it doesn't have the same capacity as the others. Maybe it is older, from a different manufacturer, etc.

PAULH

Those are good answers except I don't think a practical automatic balancing circuit has been invented yet. You can try connecting a solar panel (in sunlight) to the center battery or a 14 or 15 volt power cube to the center battery, for a few hours, while all the batteries are charging and/or at a separate time. Be sure the main charger is unplugged while connecting or disconnecting the solar panel or power cube as an electric shock is possible with some kinds of chargers. That is unlikely to solve the problem long term, so replacing all 3 batteries is likely best. Possibly switching the middle battery with one of the end batteries, in January and July each year will extend the life of your new set as the center battery is almost always warmer than the other two. You do need to charge your battery about monthly even if you don't ride your bicycle at all. The new batteries need to be charged before you ride the first time. Riding until the performance is way down will shorten the life of your new batteries. Best is to charge promptly after each bike ride. Neil

Ray;mond

I think you're problem might be that the middle battery might not have been fully charged or it might be an older battery than the other two. If you start with 3 new batteries fully charged they should all discharge at the same rate. When one battery dies, all should be replaced. You should never mix new and old batteries, this is true for 12 volt lead acid batteries and batteries used in consumer electronics. Make sure that the 3 lead acid batteries have the same ampere hour capacities. How are the batteries charged?

Roger

You can charge several batteries in series, but you need some kind of balancing circuitry attached to each battery to balance the voltages. Otherwise, they will not be equally charged and they could be damaged.

Rick

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