Join Date: Aug 2010
Charging systems and various battery styles
There have been quite a few, very informative, threads here recently in regards to engine generators, lithium batteries, solar, propane generators, etc, etc. Lots of very good information in all of them.
One thing that I think has not come up often or to sufficient detail in some of the discussions, IMO, is the affect that the battery type has on how all the other stuff works. If mentioned at all, it is usually as sidebar advantage of what solar will do, or something like that.
The focus of a lot of the discussions has been high usage of 12v power to run, mostly, air conditioning of the batteries. Of course other loads would do nearly the same thing. Engine generators are great things and can give 300+ amps to the system as long as the van is running (a totally other discussion), although a bit over 200 seems to be the norm when hot and running hard. This is still double what most of the AC units take, so the engine would only need to run 1/2 the time. All the other methods are much lower charge rates. A big shore charger is 100 amps, solar usually in the 100ah per day maximum, etc so they aren't going to give the bang of an engine generator.
What I do think is being missed in many of the discussions is that the benefits of the engine generator with lithium batteries is being confused with it's benefits with others like AGM or wet cells. With lithium, you can put in your 200 amps pretty much to full, and you never need to get full, so the engine generator will run at max output pretty much all the time, so you can replace at high rate whatever you used. This is what most folks seem to be looking at in calculating what results to expect. Unfortunately, if you are running AGM or wet cells, most of this goes out the window. AGMs will accept the big current until maybe 80% full, so all is good to there, but from there on the current drops huge and it will take another 5+ hours to fill up those batteries, instead of minutes with lithium. Put on top of that the fact that AGMs and wet cells should be taken full every 7-10 recharge cycles, to prevent early failure, and the benefits of the high amp charging fade a bit.
This is not a bashing of engine generators, but just the opposite. We removed our Onan last year and run a 250 amp alternator, 100 amp shore charger, and 300 watts of solar now. We don't try to cover air conditioning off batteries, as we never us the AC anyway. What this is has to do with expectations from a system similar to ours or others like it (most are similar, with the exception of separate or combined alternators).
A real world example would be: You have a 400ah AGM battery bank, 200 amps (average) of engine charging, 100 amps of shore charging, 200-300 watts of solar. If you run the bank down to 50% (200ah down), you will likely get 120ah back in just over 30 minutes (that will get the batteries to 80% and is 120/200 to get .6 hours). All is good to here and similar to lithium. From this point on it goes downhill as the acceptance of the batteries drops very quickly. It will take at least another 5 hours of charging to get the batteries full, and most folks will not want to run the van that long as it is very wasteful. If you have solar and good sun, the solar will be able to top you off in just slightly longer than the 5 hours, so this is where the comments about solar being useful in engine generator setups comes in (Greg mentions this often). If you don't do the longer charge cycle approx once a week, your battery life will drop very, very quickly. The boaters have proved this with certainty, as that is how most of them are using their batteries.
It is not all doom and gloom, though, just awareness for us land dwellers. You just need to get full once a week. It could be a very long drive, a short drive and good sun (in you have solar), or a short drive and shore power for 12+ hours. Not out of the question for most of us. We have run this way for many weeks in a row without any inconvenience.
Our bottom line is that we will not hesitate to run in the 40-80% state of charge on the batteries for a week+, and then find a way to get totally full for a cycle.
I urge everyone to read the threads carefully. If a poster is saying they can replace 200ah of usage in an hour and be full, they should have lithium. If they just say replace, they will have to be less than 80% full if they have AGMs. If someone says they can replace their usage to full with AGMs in an hour or two from the engine generator, they are not being accurate, as they can't get AGMs full that quickly. If someone says they run the engine until they get to 80% and then let the solar, or shore power, top the batteries off-they know what they are doing.