# Charging NiCd & NiMH in parallel

Discussion in 'Electronic Basics' started by Tim Brown, Jul 7, 2004.

1. ### Tim BrownGuest

1. In order to charge nickel based batteries in parallel we
have to use a regime that is primarily constant voltage, since we
don't have control over the current in individual cells or strings.

2. If we give up the ability of rapidly charging to a high state of
charge, it should be possible to set the charge voltage below the neg.
delta V region.

3. Once the charge current has dropped at the constant voltage level
charge can switch to a constant current of C/20 of one string/cell. At
worst
case imbalance one string charges at C/20. The more parallel cells
are used the slower this topping stage must be. A timer cut off may be
desirable.

4. Since we can't count on all the cells reaching 100% SOC the pack
should be built with excess capacity.

5. A fuse in series with each string would be desirable.

I have experimented with charging two parallel strings of cells at C/2
(total pack) up to 1.35V per cell (should have a negative temp
coefficient in a practical design), then holding the voltage there
until the current drops to C/20 of one string (C/40 of total
capacity),
then holding the current steady. Results look promising.

Why charge in parallel? Any scheme to balance charge current or
seperate cells for charging adds complexity and series IR loss. Big
cells (F size) cost more \$/Ah than C or D cells.

Tim

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