Regarding charging (batteries, not customers)

[williamwright]

In view of recent discussions here, as I drove my motorhome back from a short trip today I thought it might be worth reporting the behaviour of the batteries and chargers, so I made notes.

There are five electrical circuits.

1. The vehicle electrics (V), which are 24V. The only connection with the other circuits is that when the engine has been running for 45 seconds Aux A is connected and is charged, and the same thing after 75 seconds for Aux B.
2. Aux A is a 24V circuit. It has a 110Ah pair of batteries and can supply anything in the van except vehicle stuff. It is charged by the alternator when travelling or from the mains via a 20A 'intelligent' charger.
3. Aux B is a 24V circuit. It has a 110Ah pair of batteries and can supply anything in the van except vehicle stuff.
4. Aux C is a 12V circuit. It has an 80Ah battery and supplies all the

12V items. Charging is from a 24/12 converter that can run from Aux A or B.

1. Mains. Powered from an inverter, or automatically from external mains if available. Heavy loads (heaters) can only be powered from external mains.

When parked the van is normally on mains. After a few days the ammeters and voltmeters will read roughly as follows: V: 24.4V, -0.01A A: 27.8V, +0.15A B: 28.1V, +0.30A C: 14.6V, +0.03A

Travelling from home following the above state, after five minutes running: V: 26.4V, +5.00A A: 26.6V, +0.00A B: 26.3V, +0.00A C: 14.6V, +0.03A

Travelling from two nights' non-mains camping, after five minutes running: V: 26.4V, +3.00A A: 26.6V, +13.00A B: 26.3V, +18.00A C: 14.6V, +8.00A

Travelling from two nights' non-mains camping, arriving home after an hour's running: V: 26.4V, +2.00A A: 26.6V, +4.00A B: 26.3V, +4.00A C: 14.6V, +3.00A

Back home and on mains, after two or three minutes. V: 25.8V, -0.01A A: 28.2V, +13.00A B: 28.3V, +18.00A C: 14.6V, +8.00A

Back home and on mains, after a week. V: 25.0V, -0.01A A: 28.2V, +0.20A B: 28.3V, +0.15A C: 14.6V, +0.01A

So this shows that the vehicle is set up to charge the batteries quite slowly, and up to a certain state of charge. This seems to me to be normal for alternator charging. Charging from the chargers, even after a period charging from the alternator, is faster and to a higher voltage. What appears to be leakage from the vehicle battery is just meter error I think.

Bill

[Brian Gaff (Sofa]

Surely though there is always some drain from the vehicle battery due to all its electronics running. So that small drain may not be an error. It must have quite a complex switching set up, probably with some logic built in in any case and it would be interesting to know which circuit it uses to keep this running.

Brian

[Harry Bloomfield, Esq.]

williamwright formulated on Sunday :

If it is a modern vehicle, it will have various circuits which need some small amount of power to maintain its systems. My cars various computer systems, once fully into sleep mode, draw around 20mA. Even that 20mA pulse higher, when the alarm system has a 'quick look around'. Only older cars had no discharge at all, when parked, unless they had alarm systems.

[Andrew]

I could leave my 1998 Astra F estate (petrol), with a factory immobiliser (that shoed a slow blinking red led on the dash) for 7 weeks and it would start without an issue.

My current Astra H (59 reg) handbook says disconnect battery if vehicle is to be lft for more than 3 weeks unused. Seems a bit OTT to me.

[williamwright]

It's thirty years old! Electronics wasn't invented!

The only thing is, the 12V circuit powers the LEDs that light the ammeters and voltmeters.

Bill

[Dave Plowman (News]

IMHO. 14.6v is far too high a voltage to float a 12v battery at. But if this was accurate, I'd expect more than 30mA to be flowing.

I have a checked and accurate volt meter sitting across the battery on the old Rover. After a cold start the modern 100 amp alternator will read

14.6v, but quite quickly reduces that voltage. It stabilises at 13.8v after some time.

FWIW, I found connecting that voltmeter to a more convenient part of the wiring harness gave a very different reading to that across the battery. So ran new and separate cabling (positive and ground) just to it direct from the battery. With a relay to isolate it when the engine is stopped.

[Chris Green]

I agree, somewhere around 14.5v is the terminal (as in final) voltage to fully charge a nominal 12v lead acid battery. The charger should then drop back to something like 13.5v, 'float' voltage, to maintain the battery.

The exact voltages will vary with temperature and the exact type of battery.

[charles]

some was - otherwise you wouldn't have been able to watch the telly

[williamwright]

It was a joke!

Bill

[williamwright]

The voltage readings come direct from the battery, rather than from a point on the general circuit. I think that voltmeter might be reading a bit high actually. They all read very high in winter when the van isn't in use. I put this down to damp across the the very high value resistors in the PD networks. So that might still be a factor.

Incidentally a late addition was a relay that links the two 24V supplies when the 24V microwave oven is in use. This was to reduce the discharge rate from the circuit A pair of batteries. The micro is connected directly to Battery A because its current draw far exceeds what the ammeters could cope with. So when the micro is in use Battery A shows a charge of about 10A and Battery B shows a corresponding discharge. It's interesting that the wiring and relay resistance of the link means that Battery B only contributes 10A whilst Battery A (very thick short wires direct to the micro) contributes 30A. I didn't think it was worthwhile to improve this because it would mean moving Batteries B from the back of the van to the middle.

Bill

[williamwright]

Yes, I think I'll check the voltage with an accurate meter tomorrow. It does seem high. Of course it's the output from a 24/12V converter. Oddly it takes a long time to charge the battery, always has done, and does now with a new battery.

[Harry Bloomfield, Esq.]

Andrew was thinking very hard :

Modern cars have much more electronics systems which all draw some current when off, much more regular use is made of cars these days, so not usually a problem.

I don't make such use of my car, so it sits in my garage whilst unused and I have it plugged into a charger. The charger comes on for 20 minute each day, timed by a Smart Plug, just to keep it near a full charge.

[T i m]

<snip>

I'm not sure that's the best plan, especially if it's a reasonable current / smart charger.

Like many battery chemistries, lead acid batteries don't like being stored at full charge (the acid is at it's strongest and so most corrosive) and if the battery isn't losing much charge between charges, you may be constantly 'topping off' the charge, again, not a good idea.

So, if the battery can still easily start the car after being idle for say 4 weeks, maybe either:

Leave the charger (if 'smart') on 24/7. 'It' will get the battery to full charge and then just sit in 'maintenance mode', monitoring the voltage and only applying a small charge current in pulses when required.

or

If not a smart charger, only have it come on for say 1 day in 10, giving it a chance to fully charge the battery but not keep doing so daily.

The bottom line, you really only want to protect the battery from going flat (damaging the battery) but whilst being able to start the car when required, even at short notice.

When you dis/re-connect a smart charger it *may* go though a full charging cycle, including some time on the bulk phase.

I am about to make an 8 way charger multiplexor (managed by my home automation system) that allows me to switch a relatively expensive smart charger between up to 8 LA batteries, once a month, spending a day on each.

Cheers, T i m

[Dave Plowman (News]

Didn't realise you were that young, Bill. What did you put up aerials for then? ;-)

[Max Demian]

Actually, before about 1980, it might as well have been true as the only electronics that most cars had was the radio. I suppose manufacturers were worried about vibration, damp and temperature changes. I think electronic ignition was about the first to come in.

[Bob Eager]

I remember the good old Lucas [1] 'cutout' device for controlling the charging 'system'.

[1] Lucas, Prince of Darkness.

[Dave Plowman (News]

Not really. Alternators became the norm when power transistors became affordable. Over a decade earlier than 1980. And electronic ignition. Of course such things started first on more expensive cars and spread downwards.

Car mechanics have had many many years to come to terms with things going electronic rather than mechanical, but most have chosen not to learn about them.

[Dave Plowman (News]

I've just had the old Rover battery on charge using the Lidl charger I built into it. It's the smart one which will pulse charge a very low battery, then charge at a constant current, and change to float when full. So took the opportunity to remind me of that float voltage. Said to be OK to leave on indefinitely. Near exactly 13v.

[charles]

I added an alternator to my Anglia in latish 60s. the load broke the take off pulley on the crankshaft. I managed to get a solid one, for the Lotus Cortina - I think

[williamwright]

Do you mean prices? An Antiference X15/9K BBC and ITV aerial £9 10s. If they had a BBC2 aerial at the same time it added £4.10s.

Bill