Dan Baker wrote:
I have question around charging the battery pack on an EV from a larger pack of batteries, same voltage.
Think about what happens if you connect two buckets of water with a siphon hose. Water flows from the higher one to the lower one, until they are both at the same level. No pump is needed! If the water level is quite a bit different, the initial rate of flow is quite high. But as they near the same level, the amount of water that transfers gets lower and lower. Depending on how big and how long your siphon hose is, it can take a very long time for them to finally reach the same level.
Lead-acid batteries behave the same way when connected directly in parallel. If both batteries are the same nominal voltage, current flows from the more-charged one to the less-charged one, until they are both at the same voltage and same state of charge. For instance, connect two same-capacity batteries in parallel. One is fully charged, and one is dead. They both end up at 50% state of charge.
If you connect them with big thick low-resistance wires, the initial current can be very large! The peak is roughly the amphour capacity of the biggest battery (i.e. 100 amps if it's a 100ah battery).
But it quickly falls to a low level, as the current makes the higher-voltage battery drop, and the lower-voltage battery rises. It takes a day or more for them to fully equalize to the same state of charge.
You can limit the current with resistance in the wires. Deliberately using smaller or longer wires, for example. The voltage difference between the batteries is quite small, so it doesn't take much resistance.
If you're in a hurry, then your "source" pack can be a little higher voltage (no more than 10% higher than the "destination" pack). The current flow will be even larger, and stay higher longer. You have to monitor the current and time it carefully, so the lower-voltage pack won't get overcharged. This is called "dump charging". Done right, it's about the fastest way to charge batteries. Done wrong, it's one of the best ways to destroy batteries!
Charging this way is very efficient. The only losses are the inevitable efficiency losses in the batteries themselves. The wire between them is essentially lossless.
My questions for the list: how do I transfer the charge from the solar bank to the EV pack? Can I just use a charge cable directly and the two banks will equalize?
Yes. If they are the same nominal voltage, this is safe. You can leave the cable connected as long as you like. Just be aware that the peak current when first connected can be very high.
Or would I need to use a charge controller?
You can. In fact, *some* kind of charge control is necessary when the charging source is more than 10% above the battery's voltage.
The charge controller will inevitably lower your efficiency a bit.
a 45 amp solar controller I could hook from the main battery pack to the EV pack but I fear the input amps would exceed it?
If this is a charge controller designed for PV cells to a battery, don't use it with a battery (or other low-resistance) power source on its input. It's intended to be powered by PV panels, which are a *current* source. They can be safely shorted to reduce output. But you can't short a battery or power supply!
-- If you're not stubborn, you'll give up on experiments too soon. And if you're not flexible, you'll pound your head against the wall and you won't see a different solution to a problem you're trying to solve. -- Jeff Bezos -- Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
