Charging batteries through inverter & charger

Submitted: Tuesday, May 10, 2005 at 17:48
ThreadID: 22854 Views:13260 Replies:6 FollowUps:10
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Just kicking around some ideas for auxiliary battery setups while camping. Will eventually get a dual battery setup, but would also like to take an appropriate-sized deep cycle battery that can be removed from the vehicle and used "in-camp" for various purposes (fridge, flouros etc). I've seen a few references to charging batteries in-vehicle using an inverter and 240V charger, and was a little hazy on the maths for inverter sizing (while being aware that the inverter would need it's own wiring, and not run through a cigarette plug!).

Looking at using an AGM battery, so for arguments sake I've picked a 12V 10A Xantrax charger to get some numbers. Max output charge voltage is 14.8V, and a charge current of 10A, and an efficiency of 82%. So would this translate to 14.8V * 10A / 82% = 180.5W maximum power usage (i.e. P=VA)? So a 200-300W inverter would be fine?

If that's the case, would the max amp draw of the inverter (assuming a sine wave, say 90% efficiency), be 180.5W / 12V / 90% = 16.7A? Sounds very inefficient, but given there's losses in both the inverter and charger, might be right.

I'm hoping that on the odd occasions where we would be recharging the camp battery on the road, rather than just taking a battery along I've charged at home, a day's driving between campsites (i.e. about 8 hours on the road) would allow me to push up to 80A charge into the removable camp battery.

Lastly, where would the wiring for the inverter be run from in a dual-battery system if one was installed? Bit ignorant when it comes to this - will be my first dual battery setup. It draws power from "somewhere up front!", but assuming there's some dual battery management system in place, where do you get 20 odd amps from to run to the back of the vehicle to make sure there's ample juice? Or is the fact you've got dual batteries under the bonnet irrelevant?

I've been giving this some thought, and would rather have a dual battery and inverter setup, than a triple battery charging setup (i.e. Rotronics etc) permanently in the vehicle - will never buy a camper trailer or caravan that would require a battery bank in it, and it also makes use of a deep cycle charger I can then use out of the vehicle. Or is this a poor way of doing things?

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Reply By: drivesafe - Tuesday, May 10, 2005 at 20:01

Tuesday, May 10, 2005 at 20:01
Hi Scubaroo, the system you have described will waste a huge amount of power to gain a much smaller amount of usable power.

This system, if you went this way, would probably take you at least 50% longer to recharge the battery than it would if you used an ordinary dual battery charge controller.

Contrary to some of the stories you read on the web, you CAN fully charge any automotive battery from the vehicle’s alternator as long as you can get 13.8 volts to the battery being charged. As most vehicle ( all when working properly ) produce between 13.8 and 14.2 volts, you should have no problems charging the battery provided you put in large size cable and optionally but recommend, and a good quality dual battery charge controller.
AnswerID: 110685

Follow Up By: hl - Tuesday, May 10, 2005 at 20:32

Tuesday, May 10, 2005 at 20:32
I would not call it a huge waste of power, but it is somewhat inefficient.
The only problem with your suggestion is that a deep cycle battery connected to the main will initially take too much current if it is flat, then not enough as it is charging, so an AC charger designed to charge deep cycle and run through an inverter is not such a bad idea. It also allows you to top up the deep cycle from the vehicle without starting the engine.
I have done it this way and think it is ok.
But you will probably not be able to fully charge from flat in 8 hours (either way).
FollowupID: 367142

Follow Up By: Scubaroo - Tuesday, May 10, 2005 at 23:26

Tuesday, May 10, 2005 at 23:26
Sorry if I wasn't clear in the original post (I rambled a bit!), but I would want to have regular dual batteries under the bonnet eventually, and still keep this system in place. My need for a camp battery is greater than dual in-vehicle at this stage. So it would end up being effectively a three bank system, but only when we're lugging the third battery around while on the road for camping.

The battery would be charged off mains prior to the trip, so I'm just looking for a top-up while on the road.

FollowupID: 367173

Reply By: Grungle - Tuesday, May 10, 2005 at 22:47

Tuesday, May 10, 2005 at 22:47
Hi Scubaroo,

Aussie Swag Campers now incorporate this system to charge their AGM batteries. The use heavy cabling from the main vehicle battery through to a 1000W invertor which powers a 40A Xantrex 3 stage charger. They also have a smaller system of a 500W invertor and 10A Xantrex.


Because you should not parrallel an AGM/Gel with a lead acid battery due to different internal resistances etc and different charging profile requirement for an AGM.

Also it is faster and extends battery life.

Your figures are pretty good but always allow around 10-20% loss for cabling and connectors. With the cost of invertors nowadays (especially on ebay), you could easily get a 1000W modified signwave for around $300.

AnswerID: 110735

Reply By: Mainey (WA) - Tuesday, May 10, 2005 at 23:56

Tuesday, May 10, 2005 at 23:56
As drivesafe has said.

You can never have too much amperage available in the form of Deep Cycle batteries!

The simplistic answer is ... a solar panel and a rotronics isolator.
ok, it costs money initially, but in the bush it will work, it is not complicated and it won't over heat and break down, is reliable and the power output is known by looking at the solar regulator.

Ask any one who uses a solar panel/s if they could live without one…
bet the answer is .. no way!
AnswerID: 110745

Follow Up By: Member - Jimbo (VIC) - Thursday, May 12, 2005 at 07:38

Thursday, May 12, 2005 at 07:38
No Way.
FollowupID: 367359

Follow Up By: Mainey (WA) - Thursday, May 12, 2005 at 10:27

Thursday, May 12, 2005 at 10:27
. . . Why not ?
FollowupID: 367380

Follow Up By: drivesafe - Thursday, May 12, 2005 at 11:23

Thursday, May 12, 2005 at 11:23
Hi Mainey, I think you will find that Jimbo is agreeing with you. Jimbo is an avid supporter of solar panels.

Hi to you too, Jimbo.
FollowupID: 367390

Follow Up By: Mainey (WA) - Thursday, May 12, 2005 at 13:58

Thursday, May 12, 2005 at 13:58
Yes I concede, that may well have been his reply to ->
"You can never have too much amperage available in the form of Deep Cycle batteries"

An yes, he has all his battery recharging problems beaten due to modern solar technology :-)
FollowupID: 367411

Follow Up By: Member - Jimbo (VIC) - Thursday, May 12, 2005 at 18:22

Thursday, May 12, 2005 at 18:22

My comment was in reply to your question could I do without my panel?

My answer was "No Way".


FollowupID: 367454

Follow Up By: Mainey (WA) - Thursday, May 12, 2005 at 18:54

Thursday, May 12, 2005 at 18:54
prior to getting mine, I thought everything was working terrific, then I saw the results with a friends panel on my system and I just had to INVEST in some modern technology, is nothing like large batteries being charged 100% daily, I know that now 'cause the solar monitor tells me they are L0L
FollowupID: 367460

Reply By: drivesafe - Wednesday, May 11, 2005 at 07:41

Wednesday, May 11, 2005 at 07:41
Sorry folks but it just don’t make monitory sense. You are adding more work for you Alternator, it will cost an additional $500 +, It WILL take longer to charge the battery than just hooking the battery up to the Vehicle’s charging system.

All this to get if you are very lucky, may 5% more capacity and you may extend the life of your battery and even then this extended life still depends on how use or abuse the battery anyway.
You would be far better off putting a small portion of this money on fitting large cable and end up achieving the same thing and remember you still have to fit the heavier cable to get this system to work anyway.

As for toping up the battery without starting the motor. How long do you think your cranking battery will last putting a load like that on it.

Top up your auxiliary battery while you stuff the main battery.
Again, it’s not good value for money.

Mainey’s suggestion is also a good one, for what you are planning to spend on an inverter and a charger, why not invest in a small solar panel and then you will have charging power at all times during day light and and you can use it to keep the battery topped up when it’s not in use.

AnswerID: 110751

Follow Up By: Scubaroo - Wednesday, May 11, 2005 at 09:38

Wednesday, May 11, 2005 at 09:38
Hmm, last post disappeared.

If I were to put in a dual battery management system as an interim measure (ie just for charging the AGM while it's in the back of the vehicle), what happens when the battery is not in the vehicle and you're driving around day-to-day? Does the management system recognise there's no battery and work fine? Or does it require a battery to be hooked up at all times?

Budget just doesn't extend to a solar or three-way system at the moment, the most pressing need is to get the "mobile" AGM into use. Makes sense to invest in the heavy wiring to the back of the vehicle in place now though - that's money that doesn't need to be spent twice.

FollowupID: 367197

Follow Up By: drivesafe - Wednesday, May 11, 2005 at 10:09

Wednesday, May 11, 2005 at 10:09
Good one Scubaroo, putting in the heavy cable in the first place means you can do what ever you like later.

As for the dual battery charge control system, it should make no difference whether the rear battery is in or not. Ours don’t as the main operation is to maintain the cranking battery first and foremost, once fitted, just forget it and do with your rear battery as you want.

FollowupID: 367207

Reply By: Spade Newsom - Wednesday, May 11, 2005 at 21:21

Wednesday, May 11, 2005 at 21:21
Have been reading up on this system and their is some support for it.
Main application seems to be with 24 volt vehicles wanting to charge a 12V battery or 12 V battery bank.

AnswerID: 110911

Reply By: Outbacktourer - Thursday, May 12, 2005 at 09:57

Thursday, May 12, 2005 at 09:57

What you are proposing is not unknown and used by a few, myself included, who are unwilling to make the investment in either solar or dual set-ups, for whatever reason, best kept to ourselves. The MAJOR reason IMHO that you would run a setup like this is if the battery to be charged was some distance from the charge source and you NEED to compensate for voltage drop. This is why caravan and camper trailer setups are done this way. The invertor will work with lower voltages as input and still crank out the volts the charger needs to operate effectively. I myself carry a second battery in a battery box in the trailer charged by an Automatic 240V charger connected to a small invertor wired through to the constant 12V trailer plug. No worries. The battery is only used for lights and probably does not require much of a charge to top up. Heavier duty setups to run fridges etc use heavier cabling and Andersen plugs from the vehicle. I'm not sure about the extra load on the alternator or wasted power (through inefficiency) is an issue when you have a 100A Alternator and are running the car anyway. Don't keep me awake at night.
AnswerID: 110963

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