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Old Oct 06, 2009, 12:01 PM
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Charging series-wired packs - be careful ! ! !

Charging two batteries wired in series using two separate chargers has the potential to cause irrepairable damage to either one or both chargers as well as damage to one or more batteries connected to the chargers. Of course it is physically possible (and easy) to connect the balance leads and/or discharge leads of two series-wired batteries to two separate chargers, but a common ground can occur at the power supply inputs to the chargers. The only safe way to do proceed is to use individual "isolated" power supplies; two separate 12v batteries for example to power each of the chargers independently. If you were to use two AC power supplies, you MUST confirm using a continuity meter, that the power supplies do not share common ground while connected to the house's AC supply. The problem is, MANY power supplies do share common DC ground as the power supply manufacturers connect DC ground to AC "earth" ground internally. Common DC ground flows through your home's AC lines. Now with common ground, when you connect the first wire of the second series pack to the charger, you have the potential for a direct short across the first series-wired pack (pack 1 ground to pack 1 +). The short will not occur until the charge FETs actually energize. Then "poof", the damage is done in microseconds. This is probably the number one way that people damage chargers in this industry, and not only FMA Direct chargers. Most of the time, customers have no idea what went wrong. The original Cellpro 4s charger has no protection or warning against this scenario because it has AUTO start. Charge FET closes as soon as valid voltages are present on the charger output. The manual clearly warns customers against this situation. Please check the CAUTION box on page 2 of the following link:

For a detailed pictorial diagram of the proper and improper ways to charge series-wired batteries, check the following link:

Incidentally, our newer Cellpro Multi4 is capable of determining this common ground scenario and will not energize the charger when the condition exists.

But the simplest solution is to never charger two packs wired in series using two separate chargers. Disconnect series-wired packs before connecting to the charger(s). If you only have one charger and you are determined not to wait to charge each pack individually, consider the potentially safer approach of charging packs in parallel using a single charger. If you are careful and you assure that both batteries are discharged to equal levels before connecting them together or to the charger, you can make up a simple "y" adapter using Cellpro battery pigtails for example, to parallel charge 2 packs using the Cellpro chargers. Each node of a pack must be connected to corresponding nodes of the other pack. The packs must be equal cell count and equal capacity. The charger will see the two parallel connected packs as a single pack. If one battery is charged and the other is discharged, you can run into problems because when you connect the two packs up to your wiring harness, the full pack will charge the dead pack at high current with no current limiting between the two. After a short time, the cell voltages will equalize, but in the mean time you will be putting high charge rate into the dead pack which is generally not good for the battery's life.
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Last edited by Howard Matos; Jul 20, 2010 at 07:48 AM. Reason: change category
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