Quote:
Originally Posted by freebeard
Interesting. How many is a bank? X in parallel, in series with 8 in parallel?
Err. 8 cells in parallel is a battery, isn't it?
Does the lead/acid (or whatever) part charge in series with the D-cells or do the characteristics demand separate chargers?
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Freebeard,
Good questions but we're getting a bit off the topic. I'll try to answer anyway. Here goes.
I charge the NiMH pack as a unit, separately from the lead acids. I use a dedicated smart charger for that. A modular plug rewires that pack in series for charging because that charger doesn't know what to do with a huge 1.2V pack. It knows exactly how to charge an 8-cell pack of 1.2V x 8 = 9.6V, regardless of how far I've discharged them. They get discharged together in use, which balances any difference in charge between the cells. A nice feature.
I charge the lead acids together (in parallel) on another "smart" charger that supposedly brings them both up to full charge. It just sees one huge 12V battery of about 160AH capacity. Now and then I'll charge the 2 lead acids individually but I haven't seen them behave differently regardless of how I charge them. Most days I use about 15-25% of the combined lead acid's capacity and the charger needs 1.5-3 hours to do its work.
Normally a "battery" is made up of matchng cells in series. Your basic 12 volt car "battery" is made up of six 2V cells wired in series, 2Vx6=12V. So I'm calling my 8-unit assembly of D cells wired in parallel, a "pack". It produces a nominal 1.2V, or 1.3V when fully charged. Yes, plenty battery "packs" are sold that have their cells in series. I don't have any other name to call it so its a pack for me.
Series Parallel Circuit
My complete setup would properly be called a 'series parallel circuit'. You can google that and find various articles, circuit diagrams etc. A series parallel circuit uses some items that are combined in parallell with each other, and some groups or units that are wired in series with each other. Various ways to group and regroup the components are all called series-parallel. There are formulas and rules for calculating the results of doing this. They work for battery assemblies and for other circuits that consume power.
My setup:
Two 12V lead-acids in parallel, for a nominal total capacity of about 160 amp hours (AH) at 12V. You add amp hours when you parallel the batteries, and if they have the same voltage, that's the voltage of the combination, too. (If you put them in series, you add the voltage and the amps stays the same.) One 12V battery is about 100AH, the other is about 60AH, but they have the same chemistry so I'm not afraid to parallel the two. But since they're lead acid, I can safely and responsibly use only about 40% of that regularly, because draining a lead acid cell lower than that shortens its lifespan. So the capacity I'm willing to use is about 48AH or so.
8 - 1.2V NiMH D-cells, in parallel. These are rated 10,000 mAh each, or 10AH. In parallel, that's good for 8x10=80AH capacity at 1.2V.
Combining them:
The lead acids are wred in series with the NiMH pack.
The positive terminals from the pair of lead acids (combined) feed into one 4 gauge cable. The lead acids' negative terminals go to chassis ground. The combined positive cable leads into the NiMH pack's negative side, and the NiMH pack's positive side then feeds (via another 4 ga cable) to specific terminals on the car's fuse panels. When I check the voltage anywhere after the NiMH pack, it's somewhere between 13-14V, depending on state of charge. 14V does not last very long if I have anything switched on. It drops slowly to 13.8V, then 13.7, 13.6, 13.5, etc.
The 4 gauge cable is more than enough for the job it does. If I wanted to run the starter motor off it, I'd want something like a 0 gauge cable instead.
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