Charging one battery with another thru and inverter

Submitted: Friday, Apr 11, 2008 at 15:33
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Hi

This is because I am facing an unusual situation where the principal battery will only be used occasionally and reduced only say 10-15% from full. It is situated in a place where there is no mains electricity. Rather than crank up a gennie and so I don't have to leave solar panels lying around and I can do something else, would it be effective to charge it by leaving a fully charged battery with it, wacking on an inverter and running a 240V battery charger to the slightly discharged battery? Then i would of couse taking the draining battery home to be fully recharged on 240v.
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Reply By: Member - Davoe (Yalgoo) - Friday, Apr 11, 2008 at 15:44

Friday, Apr 11, 2008 at 15:44
if its only getting used occasionally would one of those bleep poor el chepo solar panels for recharging from supercrap do the job? at least you wouldnt cry if it was flogged
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Follow Up By: porl - Friday, Apr 11, 2008 at 16:08

Friday, Apr 11, 2008 at 16:08
Good suggestion but it's in a small shed under a very large tree and even if I could wire it through the roof it would not get any sun on the roof.
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Reply By: KSV. - Friday, Apr 11, 2008 at 16:12

Friday, Apr 11, 2008 at 16:12
Would be better to use Arrid charger - it is more effective then inventer + charger. But if you do not care about discharge "donor" battery a bit more inventor will do.
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Follow Up By: KSV. - Friday, Apr 11, 2008 at 16:37

Friday, Apr 11, 2008 at 16:37
Link to Arrid
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Follow Up By: Nomadic Navara - Friday, Apr 11, 2008 at 23:20

Friday, Apr 11, 2008 at 23:20
The Arrid Twincharge has a fixed voltage output of 14.5 V, unless you carefully monitor the battery you are charging you can overcharge it. 14.5 V is also a little too severe for gel and AGM batteries so you can easily stuff these. I atribute the use of a Twincharge to the demise of my maintenance free battery. I disconnected it long ago and my replacement battery has so far lasted twice as long

A better unit to use is the RanOx battery booster. Unlike the Arrid Twincharge its output is a proper three stage battery charger. See this link. The output can easily be selected to suit the battery specs with switches on the front panel. The maximum boost voltage can be set 14.0 to 15.5 V in 0.1 V steps, the float voltage 13.0 V to 14.5 V in 0.1 V steps and maxim output current 5 to 25 A in 1.0 A steps.

I installed one in my caravan this week. I found it extremely easy to set up and it works as promised.

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Reply By: Member - Reiner G (QLD) 4124 - Friday, Apr 11, 2008 at 20:47

Friday, Apr 11, 2008 at 20:47
I believe if you hook 2 batteries together one flat the other full that they would level each other.........or a bigger one charging a smaller one.........am I wrong.

Reiner
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Follow Up By: Member - Phil G (SA) - Friday, Apr 11, 2008 at 22:29

Friday, Apr 11, 2008 at 22:29
Reiner,

The voltages will equalise. But the State of Charge does not.

To recharge a battery, you need a higher voltage (like 13.5 upwards) for the chemical reactions to occur.
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Follow Up By: Nomadic Navara - Friday, Apr 11, 2008 at 23:01

Friday, Apr 11, 2008 at 23:01
To charge a battery fully you need in excess of 14 V. All a voltage of 13.5 to 13.8 will do is just overcome the internal self discharge of a battery.

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Follow Up By: Member - Phil G (SA) - Friday, Apr 11, 2008 at 23:32

Friday, Apr 11, 2008 at 23:32
Peter,
Yep correct answer, but that wasn't the question!
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Follow Up By: Nomadic Navara - Saturday, Apr 12, 2008 at 00:20

Saturday, Apr 12, 2008 at 00:20
Phil

I was commenting on your suggestion that 13.5 V will charge a battery. It will not. You need over 14 V to charge a battery.

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Follow Up By: Member - Phil G (SA) - Saturday, Apr 12, 2008 at 07:56

Saturday, Apr 12, 2008 at 07:56
Peter,
I think we agree here. Our difference is that you used the word "fully" and I didn't!

Can I suggest the following: A wet cell battery needs at least 13.2V before it starts accepting charge . At 13.5 volts it may charge that battery to say 60% of capacity; at 13.8V it may be 70%, but needs 14.7V to get anywhere close to 100%.

Cheers
Phil
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Follow Up By: Nomadic Navara - Saturday, Apr 12, 2008 at 20:37

Saturday, Apr 12, 2008 at 20:37
Phil

Your percentages will change considerably with different battery construction, however you seem to appreciate that you need higher voltages to charge a battery fully. (By fully I mean 100%.) My reply assumed that people wanting to maintain deep cycle batteries properly will want to charge them fully and not just to a level where sulphating will not take place (which is all alternators do with starting batteries.)

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Follow Up By: Member - Mike DID - Sunday, Apr 13, 2008 at 22:17

Sunday, Apr 13, 2008 at 22:17
A battery that is left below 100% charge will sulphate.
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Reply By: Eric Experience - Friday, Apr 11, 2008 at 21:22

Friday, Apr 11, 2008 at 21:22
Porl
Why not just swap the batteries over. Eric
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Follow Up By: porl - Friday, Apr 11, 2008 at 22:32

Friday, Apr 11, 2008 at 22:32
One battery is mine the other stationary one is not.

I agree if you connect two batteries directly together they will only equalise. but if I connect a 240v charger then I think there is no capacity to equalise as the inverter is feeding the charger which doesn't know the state of capacity of the battery the inverter is connnected to.Well that's what I'm hoping.
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Reply By: Nomadic Navara - Friday, Apr 11, 2008 at 23:27

Friday, Apr 11, 2008 at 23:27
An inverter driving a multi-stage battery charger is one way to do the job. However if you value your battery and want to use quality equipment you will find a RanOx Battery booster a cheaper and more efficient way to do the job.

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Follow Up By: wazzaaaa - Saturday, Apr 12, 2008 at 06:54

Saturday, Apr 12, 2008 at 06:54
Hi Peter,
Where do you buy these and how much are they?
The web site doesn't say.
Wazzaaaaa
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Follow Up By: Nomadic Navara - Saturday, Apr 12, 2008 at 20:49

Saturday, Apr 12, 2008 at 20:49
Waz

I am not aware of any agents at this stage. I have been in contact with RanOx since shortly before the booster was put on the market. I am not sure what the current price is, I have received mine at a reduced rate. Contact them through the "More Information" page (http://www.ranox.com.au/473/index.html) to find out the details and please report back.
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Follow Up By: wazzaaaa - Sunday, Apr 13, 2008 at 18:19

Sunday, Apr 13, 2008 at 18:19
Peter,
$350 delivered was the reply from them, they also sent me all info on installation and specs.
Thanks
Wazzaaaaa
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Reply By: Member - Phil G (SA) - Saturday, Apr 12, 2008 at 22:03

Saturday, Apr 12, 2008 at 22:03
There are other 12V powered battery chargers that are used by the Electric model plane/helicopter/model car people and are very flashy, relatively cheap and will probably do the job you require.

I have one thats about 8 years old, called a SuperNova 250S. I use it to charge all manner of NiMh rechargables, but it also charges Lead acid up to 50Ah capacity.

I've just been playing with it in the shed. I've powered it from a small 7Ah gel cell, and used it to recharge my twin N70 batteries (160Ah) to see what would happen, and whether it would work.

Well it worked very well - even though my car batteries were already well charged, it still upsized the voltage to 14.12V and was putting 4.2 amps into my car batteries. The voltage on the small gel cell was 12.37V while the charger was going.

I don't think the SuperNova are still around, but they have been replaced by fancier models. An example is the Swallow battery charger from Model Flight. For $99, its a bargain, but you can spend a heap more money for the flashy German built chargers listed on that site.

This has the ability to charge almost any type of battery (NiCAD, NiMH, Lithium and SLA), but as far as sealed lead acid batteries go, it will also charge them at up to 5 amps. Input voltage is 11-15V.

I will add a disclaimer that I haven't used a Swallow Charger but its specs are the same as my SuperNova, so I can't see why it wouldn't work.

Cheers
Phil
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Follow Up By: Nomadic Navara - Saturday, Apr 12, 2008 at 23:51

Saturday, Apr 12, 2008 at 23:51
Phil

These are similar to the Arrid Twincharge. They do not seem to incorporate any automatic switching down to trickle charging rates like the RanOx does. They are also low current devices with limited applications for the size batteries we use in our vans.

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Reply By: Bonz (Vic) - Saturday, Apr 12, 2008 at 23:05

Saturday, Apr 12, 2008 at 23:05
Just connect the two batteries together, don't fuss with an inverter or anything. The load from the batteries will be supplied ok from both batteries and there are no losses, with a charger run off an inverter I would expect in the order of 50% losses.
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Follow Up By: Nomadic Navara - Saturday, Apr 12, 2008 at 23:20

Saturday, Apr 12, 2008 at 23:20
That idea was suggested and debunked in Reply 3. It does not work. We are dealing with chemical reactions here - not simple ohms law type maths. You need at least 13.2 V before you can start to get any useful charging voltage into a lead acid battery. Below that voltage, any current that flows into a battery will just produce heat (ie will be wasted.)

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Follow Up By: Bonz (Vic) - Saturday, Apr 12, 2008 at 23:24

Saturday, Apr 12, 2008 at 23:24
I wasnt talking about charging the battery, I was talking about supplying the load from both batteries, you guys were on about charging etc I dint think theres a need to charge the other battery, just use both in parallel.

Ohms law also says that when two batteries are in parallel they supply same volts and combined current that was what I was suggesting. Y
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Follow Up By: Nomadic Navara - Saturday, Apr 12, 2008 at 23:31

Saturday, Apr 12, 2008 at 23:31
The subject of the thread is "Charging one battery with another thru and inverter" - if you read the text you will see the "and" should be "an." All the other replies are in that vein.

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Follow Up By: Bonz (Vic) - Saturday, Apr 12, 2008 at 23:33

Saturday, Apr 12, 2008 at 23:33
So all the other replies are wrong....?

I was merely offering a different solution.

Sheesh you dont know anything about meridians do you?
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Reply By: Robin Miller - Monday, Apr 14, 2008 at 13:11

Monday, Apr 14, 2008 at 13:11
No Problem Porl from the electronics point of view.

Thats what I do with my $39 laptop supply invertor with the output set on 15v and a small resistor is series to limit peak current.

I use an 18ah gel cell to run the laptop converter and its fairly portable and is great to top up bigger batteries.

Saves me some time as I have a tractor left unattended for weeks some times, and the boost gets her started without having to remove the main battery.

If you were to leave such a device connected and unmonitored for hours then you need to add a series power diode to drop charge volts to 14.4




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Follow Up By: porl - Monday, Apr 14, 2008 at 13:13

Monday, Apr 14, 2008 at 13:13
Great, that sounds the go.
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Follow Up By: Robin Miller - Monday, Apr 14, 2008 at 13:40

Monday, Apr 14, 2008 at 13:40
These laptop chargers mostly come in 3 and 6 amp versions all under $50 from Altronics/Jaycar/D Smith etc

Unlike the much more expensive Arrid types they drop a fuse pretty quickly if overloaded say by attempting to charge a flat battery and a resistor is some protection.
A common 1 ohm 10 watt resistor is usually a good starting point.

The charger I use has a visible inline car blade fuse and its easy to change or see if it is blown.
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