Battery selection advice requested

[SolarSteve]

Thanks for the follow up advice.

Here's a thought. Instead of upping the Ah of the batteries, how about lowering the charge going into the batteries? If I reduce my collecting capacity from ~93V @ 9A down to ~62V @ 9A I lose more in voltage drop as a percentage (roughly 3% vs. 2%), but I'm still at ~60V. As I understand it, that gives me sufficient voltage to maintain charging early/late in the day and suboptimal cloud conditions. It also puts the max charge at roughly 22A down from the previous 34A, which seems to be more inline with what a 105Ah system wants.

Do I have that right? Is the lower amperage sufficient to keep two 105Ah AGMs happy over the 5 day charge period?

What you can see me trying to do here is avoiding an overkill system for what amounts to keeping some beer cold This is why I'm not keen on upping the Ah storage capacity as there's a perpetual cost that comes with that. If what I've just proposed seems workable, it has the following cost benefits:

1. I only have to purchase 2 panels instead of 3, which saves me ~$180
2. I only have to spend ~$250 on a second AGM battery vs. a larger amount on any other battery solution (cost could be 2x or higher)
3. I can knock my controller down from 40A to a 30A, which in the case of Victron is a ~$100 savings

The savings "pay for" the second AGM battery, which isn't a bad thing in my mind.

Of course the downside of this is I am truly sizing my system for a specific use purpose and pattern. Occasional problems, like extended stays with crap weather, can be overcome by the use of my existing 20A generator. However, should I significantly deviate from either what I'm running or how long I'm running it for, then I could run into trouble. I'm OK with running that risk.

Steve

[sensij]

If the advice is to ditch the AGM I'll do that, even though I don't have much use for it elsewhere. Sucks to think of the money in that battery going to waste, but if that's the way it winds up that's the way it winds up.

Since you already have one AGM battery, you might as well get another and use the pair. If you can truly stick to 720 Wh of discharge daily when you are there, and with occasional use, they might work out OK for you. AGM's will take a higher charge rate than FLA, and although your system is capable of producing C/3 at STC, which is about as much as you'd want to put into them, you aren't going to produce that charge rate often enough to worry about it. 2 days usage and 5 days of charging gives them the longer absorb/float period that they'll want.

If you end up with bad weather on a four day weekend you could run into trouble, especially if that means you are stuck inside and using more power than normal.

Edit: I saw you listed the Morningstar charge controller in your previous post. You might also want to look into the Victron 150/35, about $100 less.

[littleharbor]

With a 24 volt system you will be putting around 30 amps into your battery, Two 105 Ah. 12 volt batteries are still only 105 Ah. @24 volts. Thirty amps is too much current for that size battery. You want to keep your current at 5 to 13% of your battery Ah. rating Four 220 Ah. 6 volt golf cart batteries would be a better match, although still at the top limit of charging power.

[SolarSteve]

I'm setting up a new system and currently have a brand new 105Ah Duracel AGM deep cycle battery. I need to bump up to 24v so I need a pair of matching batteries. The question is if I should buy a second 105Ah AGM or if I should instead go out and purchase two 105Ah FLA deep cycle batteries instead. Or if there's a third option I've not considered.

The system I've got in mind (discussed in another thread) is as follows:

3 x 265W panels
~93V output @ ~9A
100' 10AGW copper wire to controller
40A controller
unobstructed southern exposure for roughly 10 hours a day
summer months primary + tail end of spring and start of fall

I expect to drain about 30Ah a day for two days in a row then shut down all draw on the batteries for the following five days. This means there's seven days of variable strength charge for two days of drain. As far as I can tell the planned system has what I need to charge either FLA or AGM option sufficiently for my needs.

Thoughts about what I should do?

If the advice is to ditch the AGM I'll do that, even though I don't have much use for it elsewhere. Sucks to think of the money in that battery going to waste, but if that's the way it winds up that's the way it winds up.

Thanks!

Steve