flash gun batteries and chargers

Started Jan 12, 2013 | Discussions thread
Duncan C
Veteran MemberPosts: 7,564
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Re: flash gun batteries and chargers
In reply to b4cktr4ck, Jan 14, 2013

b4cktr4ck wrote:

Hi,

I've been reading about what would be a good charger for battery used in flash guns (One of the most recommended charger being the Maha MH-C9000).

Some comments saying that apparently if one of your 4 AAs in your speedlight has a reduced capacity then it has a marked effect on the total capacity of all 4 - so the best cell is only as good as the worst cell....

Is this still the case? that is important to get a good charger eg to show you different charge values? Without getting to specific/technical, any general advice you can give?

thanks in advance

Mike

We really DO need to get into the specific/technical details.

First point: NiMH batteries are damaged/destroyed if you over-charge them. They should be charged to 100%, no more. Charging them less leaves unused capacity. Why buy high capacity cells if you don't use that capacity.

Dumb chargers either charge for a fixed time (really, really bad)  or measure the charge in a single cell, or every other cell, and charge the other cells based on that measurement. As batteries age, their capacities change, not always evenly. By charging 4 batteries based on the charge reading from one of them, you risk over-charging and damaging/destroying 3 of the 4 cells.

Second point: Over time, NiMH batteries develop crystals in their electrolyte that make them lose capacity. Better chargers use pulse charging, which tends to break up those crystals and regain some of that lost capacity.

Third point: High current devices like flashes need lots of current and fairly high voltage in order to work. If one of the cells in a group of 4 runs out of juice, the voltage of the whole set drops dramatically, and the flash can't get enough voltage to recycle any more. (I believe I remember reading that the internal resistance of the discharged cell also increases, which reduces the amount of current that can flow through the whole set of cells. I'm not 100% positive on this last point however.)

So, all that means:

1. You need a charger that measures the charge level on each cell individually and charges each cell to 100%

2. You need a pulse charger.

3. A charger that tells you the capacity of your batteries lets you match them together, so you get the most flashes out of a set. (If you have 4 batteries that are down to 70% of their original capacity, and 8 other batteries that hold almost 100% of their original capacity, you should put all the 70% batteries together. Putting one of those 70% cells in with 3 brand-new cells that are at 100%, you'd only get 70% out of all 4 cells before the flash stops recycling. (And then if you put those 4 cells in a dumb charger, the 3 new cells are not fully discharged, so the charger may over-charge and damage them, or it might overcharge and finish off the 70% cell, depending on which cell's charge level the charger measures.)

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