# Battery charging - will this work?



## EuropeanCampers (Mar 28, 2008)

To you knowledgeable electrical peeps out there.

Prior to buying more battery power, I thought I'd test just how long I can go without EHU. At present I just have the one 85ah Gel battery. Last summer I was away with my son without EHU for 5 days but we were barely in the van so not a true reflection.

At Easter there are 4 of us away and I think this will be a good test, 3 days without EHU. How long will we last?

As a contingency, I have put a 110ah leisure battery in the boot of the MH (not wired up to the electrics). From this I have added a 500w inverter. My idea is that if the main battery runs low, I can run the hookup cable with the mains adapter to the inverter on the separate battery, thus drawing from the 110ah battery to fully charge the 85ah battery. This is only temporary to test usage and recharge if necessary.

I can't think of any reason why this wouldn't work, am I right?


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## JeanLuc (Jan 13, 2007)

It seems somewhat illogical to me, but I can see why you are considering the option.

Assuming you have a Schaudt EBL (mine is an EBL99) that draws just short of 400W.

So, 400 / 12 / 0.85 = 39 amps
(the 0.85 factor is to allow for 85% efficiency of the inverter)

If you run the 'inverter to mains' link for 2 hours, you will drain 78 Ah from the 110Ah spare battery - a 70% discharge rate, which is probably as low as you want to go.

The EBL has a maximum charging current of 18 amps, so in two hours you will put back 36Ah at most. That's not a very good return on electrical investment. I think there may be other arcane laws of electricity at play that will further limit your outcome, but I don't know enough to comment on that.

Surely better to switch the batteries if the main one runs out?

Philip


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## sallytrafic (Jan 17, 2006)

It may not fully charge it 

The 500W inverter will power your charger but will run its fan etc and so not be efficient. It will depending on your battery charger have to run for quite a few hours. 

There is the very slight possibility that your system will not be safe because of earthing requirements. ie your hookup earth should be connected to vehicle chassis by the manufacturer and your supply will be floatiing. If all you turn on is your charger then it should be safe enough.


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## macone48 (Mar 14, 2006)

The losses from using an invertor to change 12v to 240 Ac back to 12v are pretty large.

You have a 110AH battery and a potentially flat 85 AH battery that needs charging.

If you strap (pos to pos, neg to neg) the 110 to the 85 it will theoretically bring the 85 up to around 50-ish AH whilst loosing itself about half its charge. 

The 110AH will never fully charge the 85AH without external help (another battery or mains charger)

Why do you not, when the 85 is getting "low", substutitue the 110AH ??? Yes, space is probably an issue!!

All in my "very" humble electric/electronic experience.

I'm sure many others will give all sorts of further advice.

HTH

Trev


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## EuropeanCampers (Mar 28, 2008)

Thanks guys, rethink needed! I didnt appreciate the large losses or that the EBL ran at 400w as Philip mentioned.

Yes, you are right, I cant just swap them over as the 85ah is under the captains seat so a) the 110ah wont fit and b) access is a pain in the backside! I prehaps need to investigate this further though.

I knew my "genius" was flawed! :lol:


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## JohnsCrossMotorHomes (Jul 21, 2007)

GEH007 said:


> Thanks guys, rethink needed! I didnt appreciate the large losses or that the EBL ran at 400w as Philip mentioned.
> Yes, you are right, I cant just swap them over as the 85ah is under the captains seat so a) the 110ah wont fit and b) access is a pain in the backside! I prehaps need to investigate this further though.
> I knew my "genius" was flawed! :lol:


One pair of good jump leads! 

Peter


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## CliveMott (Mar 10, 2008)

To put it simply JUST USE A SET OF JUMP LEADS

C.


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## EuropeanCampers (Mar 28, 2008)

:lol: Thanks Peter/Clive. Brilliant, why didnt I think of that! Trying to be too clever!

Quick Question. What happens when I connect the 2 together? Will the power be drawn direct from the secondary 110ah battery or will it use the 110ah to put charge into the main 85ah battery and thus power the system? And will it give an almost 1ah out/1 ah in return? Just curious.

Does it matter that the 85ah is Gel and the 110ah normal wet?

Many thanks.


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## macone48 (Mar 14, 2006)

GEH007 said:


> :lol: Thanks Peter/Clive. Brilliant, why didnt I think of that! Trying to be too clever!
> 
> Quick Question. What happens when I connect the 2 together? Will the power be drawn direct from the secondary 110ah battery or will it use the 110ah to put charge into the main 85ah battery and thus power the system? And will it give an almost 1ah out/1 ah in return? Just curious.
> 
> ...


That was the theme of my post above...................... :roll:

Gel/Wet won't make any diff when parallelling. Charger setting WILL need changing.

Trev


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## RichardnGill (Aug 31, 2006)

GEH007 said:


> :lol: Thanks Peter/Clive. Brilliant, why didnt I think of that! Trying to be too clever!
> 
> Quick Question. What happens when I connect the 2 together? Will the power be drawn direct from the secondary 110ah battery or will it use the 110ah to put charge into the main 85ah battery and thus power the system? And will it give an almost 1ah out/1 ah in return? Just curious.
> 
> ...


When you connect them together the two batts will act as one. The charge levels will even themselves up. So the flat batt will charge up and the full batt will empty to the level when they are both the same.

I use to have to do a similar thing on our old van, I made a lead up with some small croc clips and some thick,ish mains cable. I use to put the extra batt on the door step of the van not too far away from the hab batt.

Richard...

Richard...


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## EuropeanCampers (Mar 28, 2008)

macone48 said:


> GEH007 said:
> 
> 
> > :lol: Thanks Peter/Clive. Brilliant, why didnt I think of that! Trying to be too clever!
> ...


Apologies Trev, didnt realise thats what you were getting at, I need it in simple terms 

Thanks to you also for trying to educate me!


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## clive1821 (Mar 27, 2010)

Hi The use of an inverter is very wasteful as already discussed, I use two batteries in parrell one 80Hh and the other 120Ah and have a 120 watt solar panel for charging on the roof which is always connected to the batteries. I use the batteries only when theres no mains, for lighting etc... but the leisure batteries supply the ignighter on the fridge, the water pump, the hot water/heating system, with that in mind, we went to north spain for two week over christmas and had no problems. and have never had any problems in the UK last year. I have found leaving my camper standing for 3 weeks the main vechicle battery was discharged, the reason is the there's a continues discharge of 550mA from the radio, alarms etc... which is logical... so have added another 100W solar panel and that charges the main battery, and all is well now. Just my ideas

Clive


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## Spooky_b329 (Jan 24, 2009)

It would be better run your 85ahr to say 50% to allow you to calculate how long you can survive, and then disconnect it, stick the 110ahr in the drivers footwell and extend the disconnected battery leads via the jump leads to the 110ahr. (effectively just swapping batteries, except the 110 presumably won't fit in the space for the 85ahr)

Connecting a fully charged battery to a discharged battery (particularly a sealed gel one) isn't good for the battery long term, although it happens between the cranking and leisure batteries with a basic split charge relay charging system anyway.


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## DABurleigh (May 9, 2005)

"although it happens between the cranking and leisure batteries with a basic split charge relay charging system anyway."

But via long often not too thick cables, hence resistance, hence current limit. That's also why dissimilar vehicle and leisure battery construction doesn't matter as much as between are side-by-side leisure ones.

Dave


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