Re: ONAN replacement or not?

Scott SV Tengah


If I were you, I'd save the money on a new genset and put it into a good sized lithium bank. My experience is the same as others where I rarely use my genset despite using around 200-250AH a day - I literally have it on my calendar to run it to keep it from dying from non-use. I routinely run the Bauer 2 on battery - in fact, I have never run it on genset because I don't want to deal with potential issues with exhaust getting into my scuba tanks. We don't use AC much, but it easily runs all night on battery when we tested it. The aft Climma plus the raw water pump draws around 1kw I think.

My reading of the Firefly materials when I was considering them highlighted a few things to me:

They still have the charge efficiency issues of lead batteries. Sure, it's a bit better, but they waste a lot more energy when accepting charge. 10AH from your charger does not equal 10AH increase in battery charge state.  You lose around 20%, I think.  In my 2 years with lifepo4, I'm averaging 97-98% charge efficiency. That means you'll be running your genset more.

Further look at these charts from a company trying to sell you the batteries!

Charge acceptance drops off quickly after 30 minutes while you don't get a full charge until a bit over 3 hours. Sure, it's better than regular AGM, but the point is that you'll be running your genset more at an unhealthy low load. My 2 year old lithiums take the full 200amps until well into the 90%+ SOC level so if I were to run my genset, it's not for very long and at a healthy high load.

Discharge inefficiency goes up with higher draws.  Like most lead batteries, their amp hour rating is based on a 20 hour discharge rate. Try to run a scuba compressor and see how that affects your total available amp hours. It has virtually no impact with lithium. A 100ah lithium battery will last 10 hours at 10amp draw and around 1 hour at 100amp draw. Lead tech results in big drops in actual capacity when the discharge rate goes up. Therefore, you'll be running your genset more on these high discharge rate items.

Further, if you look at Firefly's own voltage vs SOC tables, you'll see that at low SOC, voltage sag is significant and therefore your amps go up. That makes the claim that you can draw them to low SOC a bit misleading. That same Bauer compressor that takes 2200w/26.5v = 83amps on my lifepo4 system will take 91-92 amps if you're hanging out at 40% SOC. And that is resting voltage for lead. I believe it would drop much lower at that SOC under such a high load. And because of the near non-existent discharge inefficiency of lithium, that high load has little to no impact on the battery capacity or voltage. On lead batteries, it has a huge impact, so I doubt a 116AH firefly battery would last 1/2 hour with a 91amp draw. 

Bottom line - you'll run your genset more.

I remember we talked about this before and I was telling you not to believe all the marketing material and that I thought lithium was the better option in the long run. Now that your decision criteria includes whether or not to buy a new genset, if I were you, I'd be nursing the genset + lithium vs. new genset + firefly.

I know you don't like solar arches, but be aware that deck mounted solar does not output anywhere near their rated watts. Look at any of the temp vs. power output charts and you'll quickly see why.

2007 A54 #69
SV Tengah

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