Parallel charging with isolation diodes? - RC Groups
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Jan 05, 2003, 12:17 AM
Registered User

Parallel charging with isolation diodes?


Why isn't it possible to parallel slow or trickle charge packs with the same number of cells if a diode is included in the lead to each pack to prevent cross discharging?

Hank
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Jan 05, 2003, 04:26 AM
Registered User
Cross discharge is not the problem. If you connect 3 x 7 cell packs in parallel, one at 7V, one at 8.4V and the other almost full at say 10V what is the voltage of the single charging supply and how much current is going into each pack ?

Steve
Jan 05, 2003, 02:10 PM
Registered User
The voltage of the supply will always have to be slightly above a fully charged pack. Consequently, the lower the pack's voltage from fully charged, the more charging current it will get from the charger. Which is what you want.

Hank
Jan 05, 2003, 05:38 PM
Registered User
Sorry but this is basic circuit theory. If the charger is at a fixed voltage e.g. 11V are you trying to tell me that the wire (or diode) to one battery is dropping 4V while another is dropping only 1V ?

Electricity doesn't work like that and anyway chargers do not have regulated voltages, they try to maintain a constant current.

Feel free to try it but the result will be that you will never have any idea if any of the packs are undercharged or horribly overcharged nor how long to leave any of them on charge. Not a good idea.

Steve
Jan 05, 2003, 06:22 PM
"Simplify, then add lightness"
If the packs are identical except for the state of charge, and are connected directly together, the voltage at the connector will be the average of the voltage of the 3 packs. The packs that are above that average voltage will be supplying charge to those that are below and the voltage levels will eventually equalize, even if no charger is attached. The problem is that the current flowing from the high pack to the low pack is limited only by the internal resistance of the packs and could be quite high. A diode in series with each cell would prevent this current flow from pack to pack, but you wouldn't want to fly with the diodes in. With the diodes in, the lowest pack would be taking all the charging current, so you would have to limit the charging current to a safe level for one pack. It would be quicker to charge each pack individually, until they are at the same level.

Jeff