Monday, 4 February 2013

Deja Vu

In doing more research, I have managed to actually find a post from myself. In which I seem to have been asking the exact same questions before, and coming to the exact same conclusions. I guess you could ask, 'why haven't I learned?' I guess I have learned a bit, but clearly forgotten a lot as well in that time.

The posts were from the SBMCC (Self Build Motorcaravan Club) and dated the 7th August 2010. So around 2.5 years ago. I've copied and pasted them here, as you need to be a member to view them on the original site:

Hi all, I've got a question about battery charging and smart chargers... 
I've got 460Ah of battery on a 12v system (4 x 115Ah batteries) primarily charged with a 50A sterling B2B. So assuming I took it down no further than 50% it would take at very best scenario 4.5 hours to put 230A back into it assuming my alternator could put 50A in (no idea what it actualy puts in) and the charger gave it 50A continuously.
Now, I know smart chargers (mains or B2B) have several stages and start with constant current and then switch to constant voltage. According to Battery University the constant voltage stage is very necessary in order to make sure the batteries can hold the charge....
... so does that mean then unless I did a solid 4.5 hour drive (or I guess more like 10 hours, as its unlikely to get the optimum) I would never end up fully charging the batteries properly. 
Ie. if I did 5 drives each of 2 hours on different days, would it never do a full charge cycle? How do the chargers know how long to spend on each cycle? I presume they must monitor the resistance and voltage of the battery? So does that mean if the battery was very low it would take, say, 10 hours to do the full multi-stage process.... yet if the battery was only slightly discharged it would do the full cycle, but quicker?
The reason I'm asking all this, is whilst I have this massive battery bank, its has (especially recently) seemed to be much lower capacity than I would expect. I'm thinking this could be because the van only gets driven short hops, and hence the battery never gets a chance to have a complete full multi-stage charge and only gets the first stage of the charge repeatedly. Obviously age of the batteries is a factor (these are probably 3 years old now, but have been looked after fairly well). 
I've not got a mains charger installed in the van as we very very rarely ever stay on sites, so nothing to hook it up to. I have however just run the extension lead outside and plugged it my portable multi-stage 20A RAC charger to try and do a long full charge overnight to see what happens, as I've just uprated some of the cables in the wiring.
So seems I was already thinking that lots of small trips (ie small charing runs on the B2B) were conducive to good battery health:
I suspect what is happening is that I am doing lots of trip sin which the charger does stage 1 (constant current) but never gets to stage 2 (constant voltage) and hence it never gets to 'properly' charge the battery and so the battery always seems to be lower capacity even though I keep thinking I've put a fair bit of charge into it.
...and I seem to have been experiencing the same voltage drop symptoms as before:
Another data point. I've just gone out and checked the batteries. After taking the surface charge off by leaving around 3A of lights on for half an hour, each of the four batteries, measured individually is exactly 12.80V. 
However, when I put a 7.3A load on (halogen lights, compressor fridge, and 200W invertor running laptop)... admitedly an abnormally high load, the batteries read 12.54V at the battery terminals. They read 12.32V at the distribution point, so my cable upgrade has certainly helped with the voltage drop between the batteries and the distribution point. 
However, my question is.... is a drop from 12.80V -> 12.54V on the battery terminals 'normal' for drawing a 7.3A load from a 460Ah battery bank? When I switch the load off, the batteries return back to 12.80V (after about 5 minutes).  
So, hopefully now with building this battery monitor I won't be asking the same questions YET AGAIN in 3 years time ;)

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