Deep cycle or starter battery charging issues

TURNKEY wrote:
OK...I've been doing some thinking and digesting of information. To sort of recap 1)The bigger the battery bank the faster you can recharge (given a charger that is capable)

Yes. If you can send X amps into your bank, and you double the bank, you can send 2X amps into it.

2)True deep cycle batteries are more suited for the type of discharge that camping off grid causes.

Yes. DC can go to lower voltage without damage for more cycles.

3)Starter batterys are designed to discharge a lot of amps quickly, deep cycle are designed to discharge more slowly.

Yes.

SOdoesn't the make up of a deep cycle battery preclude fast charging? If it can't get out fast how can it get in fast?????

In a sense, it does - you can't recharge a single DC battery at 600 amps. It won't accept that, but so what?

You are mixing/confusing your definition of "fast." The standard SLI battery in your car (Starting Lighting Ignition) can easily dump 600+ amps into the starter for a few seconds. That's far less than a single AH of battery use. If you wanted to fast charge at 600 amps for a few seconds to replace the AH you used starting, you'd want an SLI battery. Of course, no one does that. The only reason the SLI battery can accept 600A is that it has to be designed to discharge at 600A. No one actually ever uses the capability of recharging a tiny amount of AH at that incredibly high rate. Youd have to own an expensive charger capable of charging at that rate, and then you could only run it for a short while. Even the SLI battery would be damaged after more than a short while recharging that fast.

You want a battery bank of capacity C in AH that isn't damaged when discharged about 50% to do what you want to use it for. That's the usually recommended minimum size. Then you can easily charge that bank of capacity C at the rate of C/8. If you are aggressive, you can use C/5 and if extremely aggressive at C/3. Choose one to get your maximum charge rate when charging in bulk mode.

Note that at the top end of the recharge process, your charge rate slows down. Batteries last longer when fully recharged, so you have to decide if you want to run your batteries at a lower average to improve charge rate and decrease charge time. It's a tradeoff. I prefer to limit recharge to about 90 - 95%. The higher the average SOC (State of Charge) you run, the more time you spend at the slower charge rate and the more you want a bigger bank to minimize recharge time.

As to AGM. You will read that they can accept recharge faster. It's true, at least in theory, but at least as I see it, their best advantage is when you can't vent your battery storage area or when you have to mount them on their side for space reasons. You still don't want to discharge too far and for chargers that I looked at - 90-100A - that aspect helped me not at all as compared to a conventional deep cycle flooded lead acid battery. I suppose if you wanted to go to extreme charge rates above that level, the AGM charge rate advantage might factor in, but with the charger I selected and 80A I'm only at around C/5.8. I could go to 150 amps if I wanted to be more aggressive and should still get good service life out of the batteries.

Remember my initial criteria of wanting to replenish my day's draw in as little generator time as possible.

That's exactly what I wanted to achieve and did achieve when I recently doubled my deep cycle bank size and upped my converter size. I was able to double my recharge rate, halving the recharge time and giving me more capacity to begin using the inverter more effectively. It allowed me to make more effective use of my generator power and reduce fuel cost significantly.

Here are some sample recharge charts from my system starting at 50% discharge on a 460AH bank (four 6V Sam's Club):

You can read more in this thread:
http://forums.trailerlife.com/Index.cfm/fuseaction/thread/tid/26139699/gotomsg/26355953.cfm#26355953