Best deep cycle ?

tplife wrote:
While certainly a top-line flooded battery, Trojan (and other manufacturers) offers AGM options that are far superior to flooded batteries - if your budget can afford it. Like any other piece of high-end gear, if you use it, you will get more than the cost out of it on the back end for cost-efficiency.

Battery Type Comparisons

I read this article. He compared many battery types (FLAs, AGMS, etc.) and came to the conclusion that AGMs are best for his use on a boat.

However, I had serious doubts about that conclusion.

1) He did a comparison of energy vs battery volume and found that flooded lead acids (FLA) specifically, Trojan T-105s were by far the best.

2) He did an energy storage vs. weight comparison and again found that FLA/Trojans were by far the best

3) Then he did a purchase cost comparison vs capacity. Again the FLAs won.

4) Finally, he did a lifetime total cost comparison and found that AGMs won. The method of comparison, however, was odd. He assumed you would run an engine to charge the batteries and compared the cost of running his diesel engine to charge the different batteries. It's expensive to buy gas/diesel to run an engine to charge with the alternator, so this analysis made the most important factor the charge rate - i.e. the size of the engine alternator and its current output.

The faster you recharged, the less you had to run the engine and the lower the cost. That might apply for a boat, but might not apply to your RV. It also might not apply if your charge system can't get into the above 100A charge rate region where the advantage is greatest. Finally, he ignored the decreased charge rate after you leave bulk rate. He assumed the bulk rate charge would continue through the charge cycle. Many of us spend a lot of time in absorption, not bulk, so even with a large charger, his analysis was off. It doesn't shorten charge time to have a big charger if the battery won't accept the high charge rate the charger is capable of.

Further, he had some comments about charging and discharging that I didn't agree with. There was a lot of discussion of Peukert, but it wasn't well thought out IMHO. The Peukert exponent is specific to a constant discharge until a certain voltage is reached during discharge. In reality, the Peukert exponent has two factors - one is the energy loss due to internal battery resistance - the I squared R power loss. The other, however is not a true energy loss. The battery voltage will recover if allowed to sit, and some energy appears to be lost can still be recovered if you are willing to draw out the last bits more slowly and after a recovery period. He heavily weighted the lower internal resistance of AGMs in a way that i don't think RV battery usage merits.


Finally, he disagreed with another expert on battery usage in boats who recommended charging in parallel and discharging individually, instead of in a parallel bank, where possible. I agree with the other expert. There are good reasons why discharging a parallel bank is harder on the bank than charging a parallel connected bank, and he didn't seem to understand that. (I don't do that - I discharge in parallel, but that's because it's hard to set up to discharge individually. Nonetheless, I understand why it's easier on the batteries to discharge individually, where possible.)

In general, he had lots of good info in that link, but I didn't agree with the conclusions and didn't trust his understanding of the data he posted. Still, it's worth a read.

My personal opinion is still that FLAs, such as Sam's Club 6v's are the best bank for the buck. Some of us are willing to pay more for less hassle - no water checks. Some need the reduced gas venting of AGMs or the ability to mount them on their side, etc., and some can use the faster recharge rates if they have a small battery bank and a big charger. However, for me, the bang for buck is the prime concern and FLAs can't be beat.