After having spent all this time playing with a solar system and built an array that can push out 180W of electricity, it became clear that my 8 year old (and heavily abused) 70Ah leisure battery and it's new-ish (but rapidly being ruined by deep discharging) 70Ah car battery were due for replacement with something more suitable.
A couple of weeks ago we were driving home from my parents house and spotted a caravan showroom. We stopped to look at some stuff in their shop (12v gadgets basically). I noticed that they had cheap leisure batteries but was still toying with the idea of getting a pair of Elecsol carbon fibre batteries as they are supposed to last up to 1000 deep cycles and resist sulphation if left flat. But they are expensive.
I worked out that for my largest load of some 230W inverter mains I'd actually need four 110Ah batteries as they are only rated at 110Ah for their C/20 discharge rate. The inverter draws about 10A per 100W of 230VAC mains used so at my load it would draw about 23A. For the battery pack to comfortably deliver it's rated capacity run-time I'd need to keep the discharge below C/20 or 5.5A for each 110Ah battery. So four batteries was close enough, delivering a C/20 rate of 22A.
In theory, I should be able to run this load for 20 hours but that would run the pack flat and quickly damage it (as they are regular flooded cells). The most I've used the AV kit that generates this load is about 4 hours in an evening and that completely killed the two old batteries - a) because they had to deliver more than three times their C/20 rate and b) because they weren't in great shape to begin with.
With 440Ah on tap, the four batteries should be able to deliver four hours of the load and not be discharged by more than 20%. Even if I have a couple of cloudy days with not much charging, this pack should hold out for at least three days before getting 60% discharged (still not too dangerously low).
The clincher was haggling with the store owner... He wanted 70 Pounds per battery and 9 Pounds for the snap-on terminals but as I wanted four of everything he let me have the batteries at 65 Pounds each and the terminals at 7.75 each, saving 25 Pounds. The Elecsol batteries are about 110 Pounds each...
In selecting the batteries from the pile, I made sure they were all exactly the same age as they have date of manufacture stamped into the plastic on their tops. Some in the pile were a couple of months older. I'm not sure it makes a huge difference but I'd read somewhere that it's best to get batteries that are no more than a few months different in age and to use them and cycle them together from the beginning or else they can get unbalanced if you put a new battery in an old group.
The silvery things are the carry handles that neatly snap flush into the tops of the batteries. Behind the pack you can see the 1kW pure sine inverter.
These snap-on terminals are pretty good - a Swedish make. They have big screw down pressure plates for the wires and I could easily get the 2AWG inverter cables plus other pack link wires in there - unlike the regular car battery terminals that really were a pain to get even just the inverter cables into the holes and then the screws chewed up the copper. The snap-ons also have built-in colour coded insulator tops.
For the pack links I used my 1mV/A shunts on the positives so I can measure the individual battery loads and on the negatives I used shorter lengths to keep losses down. Ideally, I should have wired them all in a star formation so that all the batteries have the same resistance path to the main load (the inverter) but I compromised by putting the load on the no.3 battery and the solar charge leads on the no.2 battery (it wouldn't fit on the no.3 terminals that were too full of wires with the inverter leads and link wires and the inverter DC isolator relay feeds). If I get bored of monitoring the currents between batteries and they start to get out of balance a lot due to the different resistances, I may try putting a fat loop between the no.1 and no.4 batteries as these are the ones furthest from the charge / load points on the busses. The end battery terminals have plenty of space left in the clamps so it won't be hard to get the extra wires in there.
The pack fits quite well behind the armchair by the patio door but I wonder if they will gas too much and risk an explosion... The old leisure battery used to draw more current when fully charged than the car battery and used to gas so much that it was always gurgling (like my stomach after a heavy meal) but I've read that old batteries are like that. We'll see tomorrow - the forecast is for sunny weather so hopefully the full 180W of PV will be brought to bear on the pack.
The patio door has a vent strip above it that can be opened and closed and lately I've kept it open to let a little draft in just above the batteries. Not sure what I'll do in winter when it's too cold to leave it open.
