For the record I’ve known there was a small problem with the way my batteries were connected from the moment I switched on the first appliance.
I have two voltage readouts on the wall of the main cabin: The top one goes to the main battery bank, which powers the inverter. The bottom goes to the secondary bank, two older batteries that – until this morning – powered the cabin’s 12-volt lighting. The problem I mentioned above is that I imagined these two battery banks would be completely independent of one another but since I powered them both through one of my two charge controllers, there was unexpected feedback: A load on one bank showed up on the other.
Until this winter it was an irritation but not really a functional problem. But then an extended gloomy period that put real stress on the batteries affected the way both banks behaved; at least one bank showed signs of damage but it showed up on the readouts of both. Not long ago a second gloomy period stressed the batteries even more, and the problem became more pronounced.
The main bank is four nearly-new T105s which should be able to shrug off that rate of discharge many times in their lives. The secondary bank is 2 much older T105s that have been neglected, sulfated, stored discharged for extended lengths of time and generally the fact that they still work at all is a tribute to their creators. So I had a pretty good idea where the problem lay.
Note that the photo above was taken at about 5:30 this morning. The following photo was taken about 20 minutes ago, say quarter to 11, after I disconnected the secondary bank from the charge controller…
The problem with the secondary bank is worse than I thought, far worse, and the problem has been masked all this time by the feedback that was giving me false voltage readings for both banks. The main bank has been propping up the clearly failing secondary bank. Even when I knew or at least suspected I was getting false information I didn’t know how false until I completely disconnected the banks from one another and had a look that was unbiased by the feedback. In theory I could have harmed my new batteries if I’d let things carry on that way.
So will you go back to a single battery bank?
Already have.
Joel, there is a simple fix for this if you can tolerate a loss of 0.7 volts between the charger and the second (older bank). I can send you some nice fat honking diodes that you put in line with charger wire feeding the + line to the weaker battery bank. That allows current to flow TO the bank but not backfeed to the other battery bank. Surprised if you don’t already have such in line.
How many amps at max charge on a brilliantly sunny day goes from your solar array to the chage controller then to the battery packs? I figure it is probably under 15 amps but don’t know for sure as I have not worked with these systems. I am sure you know the current flows from —>I—- through the diode. đ
Correction, to not backfeed the older unit, the diode would have to be in the larger one, or just do both lines, that way neither bank can backfeed the other. The downside is that silicon diodes cause a 7/10 of a volt drop and the heat to disspate will be the current at max flow times that 7/10 of a volt (Volts Loss X Current = Watts of heat to dissipate) If your charge controller is set to 14.5 or 15 then the real charge voltage would be 0.7 volts less at the battery terminal.
We are only looking at the charging circuit side. No effect on the side that draws current from the batteries. Not too hard to make up a couple with heatsinks and mount them on a wood or phenolic board. Let me know.
Terrapod, it doesnât matter any more. Upon inspection those two older batteries are for the recycler. I only rigged things that way because I couldnât bear to shitcan them while they still worked but they were never really necessary. So I wonât replace them, and that frees up the space theyâre taking. Thanks for the thought, though.
Ah, well, it was fun to play engineer for a few minutes. Keep an eye out for more batteries though, by your own rules, the more good batteries on the system the lower the worry quotient. For grins and giggles I went and looked at what those puppies cost and Amazon has me scratching my head.
One guy sells one for 400+, another sells you 4 for near a kilobuck and yet another guy will sell you 6 for that same kilobuck. What the heck is going on with this? Mind I only looked at “free shipping” listings.
https://www.amazon.com/s?k=T105&ref=nb_sb_noss
Nice big honkers they are. Will keep an eye out for low mileage used ones at auction, never know what might turn up.
Oh my god, no no no. T105s cost $150 brand new, or did six months ago. I bought my four new ones locally, no shipping involved.
You don’t think that Landlady would enjoy carting a trunk full of those thingies out to the Lair sometime???
}:-]
I did a trailer mounted solar system that uses Rolls Surette L16 type batteries. About twice the capacity of T-105s, but at about 2.5 times the cost. They’re absurdly heavy, and hard to ship. Got 4 by truck, and the tops had sulfuric acid splashed all over them. I was fond of those overalls…
OTOH, they have a lot of extra liquid in them. I’d neglected to check water for a few years and they were OK, though it took a gallon and a half to top the four batteries off. (System sees very little activity–backup for power failures.)
The big ones can be had through Northern Arizona Wind and Sun, though I don’t think they stock, but dropship. Url for amusement’s sake: https://www.solar-electric.com/
Those big Rolls Surrette batteries are popular in solar applications. My limited experience with them suggests they can be more long-lived than T105s when not heavily used – Ian has 8 that have lasted 11 years so far though under little load – and they do have bigger electrolyte reservoirs which lets them survive a bit more neglect. In heavy use they donât seem more durable than T105s which are, as you say, considerably less expensive but require more regular maintenance.