Battery system for ResMed S9 - have I got this right?

[OMyMyOHellYes]

I *think* that if I hook this battery straight to my car's 12v socket, it'll at least begin to recharge, won't it? And couldn't overcharge because the car won't supply enough voltage to take it too far? This ought to be enough to keep topping it up if we do go away for longer?

Doubtful, unless you are running the car - and that would be a terribly inefficient and expensive way to charge the battery...

Electrical systems in cars are of course 12v systems. Your battery, new, at full charge is probably pushing out 12.5-12.8v MAX. (Keep in mind that even as good as they are, the 12v AGM batteries lose their capacity over time, so a three or four year old battery may only charge up to 12.3-12.5v depending on how it's been used and how well it has been maintained). Fully discharged it will be around 11v output. Not likely really enough differential between a fully charged car battery and your AGM. All you would really be doing is hooking two twelve volt batteries up to each other. It would drain the more fully charged battery without fully charging the other one. And if your AGM battery was at a higher state of charge, it may discharge it. And even if you did manage to get a little bit of charge back in the AGM battery, your car battery may not have enough "oomph" left to start the engine.

OMM

[Paptillian]

And even if you did manage to get a little bit of charge back in the AGM battery, your car battery may not have enough "oomph" left to start the engine.

Eh... just push it down a hill and put it in gear.

(Okay, although it does work, I don't condone such risky maneuvers!)

[JamieWakeham]

Ah - should have said I'd only do this if driving somewhere! A frequent pattern for longer climbing trips is that we set up camp somewhere for a few days, but drive to a different crag each day. It seems that the socket will deliver about 14.5V when the alternator is peering it, which is why I thought that it be a good way to top the AGM up if we were driving for 30 mins or so.

[JamieWakeham]

OK, after a load more reading around the subject, this is what I'm going to try. I'll go for the 26Ah battery. I'll charge either from my old car battery charger, or direct from the car's 12v socket, until a voltmeter reads about 14.2V. This is replicating the first stage of a three stage charger and should leave the battery at about 80%.

Even if we use it for two nights with no more charging, we should still be at (26*0.8)-9 = 11Ah minimum, so about 45%, which is as low as I want to take it. As it's AGM, I think it should be perfectly able to withstand cycling between 80% and 45% for a good few cycles, and as it's a much bigger battery than we actually need I don't mind losing that top 20% of capacity. Equally it should be able to withstand the slightly less-than-optimal charging. We'll probably only do this ten times a year, so even if I kill it after 100 cycles we've got a decade out of this very cheap solution.

I'll report back whether this set-up works! Thanks for all the advice.

Jamie

[alby_c]

Sorry to make it harder - but...
The idea of using your car to charge the battery each day is great - but -
Check the wiring, voltage and current draw of the 12V cigarette socket in the car, should be in the owners manual or easily found via google. these are typically run with very light wiring and you may find yourself blowing its fuse if your planned draw exceeds the specs. If it is not fused you could set the wiring insulation on fire - be very careful, this could be very risky with a standard cigarette socket.

I would (did) pull new wires of the right gauge straight from the car battery to a new heavy duty cigarette socket, preferably in your boot or back seat area. It is really not that hard to do. Definitely fuse it - if you don't the insurance companies could go a little funny on you. 12V DC can and will heat wires very fast and set them on fire. Do a quick google search on caravan fires and it should scare you - it did me.

If you are not confident with wiring gauges and working out the current capacity - get an Auto electrician to look it over for you, but you will need at least 12G (8 better) imho. (smaller number - thicker wire)

I ran extra sockets in the 4WD with a heavy grade sockets and 8G wiring (overkill but then, I like my truck)- 40A fused to run the 12V fridge and put in an anderson plug for putting a lead with the alligator clips on if I want to charge another battery, or hook up the caravan and charge its battery. The 4WD also has a deep cycle battery for the fridge, and other uses. I like my comforts camping but then I am older and have done the rough style already.

As far as the old charger - no reason except you will probably not get a full charge and likely shorten the battery life a bit - overheating in charging is the real killer of batteries so watch that. But as you have indicated you can live with that - go for it - your money after all.

[JamieWakeham]

Thanks, Alby - that's something I'll look into. My X-Trail manual says it can deliver 10A at 12V and has a 10A fuse; I'll pop an ammeter into the system and see what it's drawing. If it's too high, I'll re-wire a heavy duty socket. Or possibly just forget about recharging from the car at all - thinking about it, it's pretty hard to envisage not being able to use the mains at least once every three days.
Cheers
Jamie

[alby_c]

Cool- A 10A socket that is fused is ok - just get a spare fuse just in case (it probably came with a set inside the cover). I had visions of a standard older sedan. The Nissan X-Trail is designed for you to use the sockets for more than just charging phones and ipods. You could probably run a fridge or an air pump off it as well if you wanted to, at least until the battery went flat or you ran out of fuel.

go for it

[archangle]

If you do any kind of wiring directly to a car battery (or an AGM or marine battery), be sure to put a fuse right at the battery. At BOTH batteries.

Charging something like an AGM or marine battery from a car battery, even with the engine running is problematic in terms of doing it "right," but it will probably work. You might not get full charge. If you do it a lot of times, you might reduce the battery life. It may also take a lot longer to get a full charge than you would think.

[JamieWakeham]

So, in case someone else researching the topic comes across this thread in the future - this seems to work. We've used the battery system for two two-night trips now - I've yet to try charging from the car as we have enough capacity to run two nights without a recharge.

For the benefit of that hypothetical future viewer, the converter (*not* inverter) was about £70 from realsleep.co.uk, and the battery we used was a 26AH AGM deep cycle which cost us about £55 from RS. If your predicted draw per night (use the pdf linked earlier in this thread to check, again making sure you're looking up the value for a converter and not inverter) is much greater or you want to go more nights without charging you will need a bigger, heavier (ours is 6kg) and more costly battery, but RS seem to have a wide range. Just make sure it's AGM and deep-cycle.

What I don't know yet is whether my skimping on a proper three-stage charger will shorten the battery life, and to be honest I won't know that for a few years at our anticipated usage rate. If you intend on using this system frequently it's probably worth spending ~£30 on a proper charger. If you want to skimp like us, I've been taking the car charger up to about 13.8V - as soon as you disconnect the charger the terminal voltage will drop and stabilise at about 12.7v (which indicates ~80% charge for an AGM cell) within an hour or so. Note this is a bit lower than the 14.2v charging voltage I predicted above. I'm sticking with slightly lower charge voltages as I suspect that overcharging will kill the battery faster than cycling it down to a lower percentage in use. The charging process draws about 2A and takes about three hours after a weekend's use. Good luck!

[bwexler]

If you look at lifeline battery web site, they provide a good bit of educational info. They include a chart showing how many cycles (charge/discharge) to expect from their batteries at various stages of discharge.
For charging while traveling you might arrange a bracket under the hood to hold your AGM battery and use short jumper cables.