Charging 100AH AGM battery whilst under load

Hi all,

Have a 100AH AGM battery setup to run a fridge and lighting in a tandem trailer used to cart catering equipment to fire crews on the fire ground. The battery should have enough charge to run the fridge for approx 8hrs, the idea being that the setup will be run from 240V genset once on location.

What's the technicalities of connecting a 240V battery charger (e.g. Ctek) to charge the battery whilst it's being used to power 12Vequipment? Can it be done? Will it cause an error? The fridge is fairly simple to change over to 240V however at this stage the lighting is all 12V and would always need to run from the battery and not 240V?


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Reply By: Sand Man (SA) - Tuesday, Apr 09, 2013 at 07:10

Tuesday, Apr 09, 2013 at 07:10
Hi Nick,

Absolutely no problem at all in charging from a 240v charger whilst under load.
This is no different than many of us do by charging from the alternator while driving and running the fridge at the same time.

If possible, it makes better sense to run the fridge from 240v when available as there is much less load on the battery, but all equipment can be run from 12v if necessary and the battery supported by the 240v charger, or even solar panels.


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AnswerID: 508519

Reply By: Member - Peter H1 (NSW) - Tuesday, Apr 09, 2013 at 07:39

Tuesday, Apr 09, 2013 at 07:39
Make sure all the lights are LED's as well.
AnswerID: 508520

Reply By: Member - John and Val - Tuesday, Apr 09, 2013 at 07:42

Tuesday, Apr 09, 2013 at 07:42
Hi Nick,

Depends on the type of fridge. If it's a 3way type (capable of running on gas) it will draw about 12-15 amps continuously. A 100 Ah battery has the nominal capacity to almost handle this for 8 hours, but in practice it won't, and will have a pretty short life, even if recharged fully without delay after each discharge.

If it's a compressor fridge it will draw an average of maybe 2 or 3 amps and a 100Ah battery will handle it easily for a lot longer than 8 hours.

As Bill says, no problem charging while drawing power. In fact the current will be coming from the charger at that time, which does reduce the amount flowing into the battery, otherwise no effect.


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AnswerID: 508522

Reply By: Bushranger1 - Tuesday, Apr 09, 2013 at 07:59

Tuesday, Apr 09, 2013 at 07:59
G'day Nick,
I run my Engel fridge & 240 Lighting (thru Inverter) at my Bush property using the same setup as you. Run the Gennie every 2 days (when I am out walking in the day) thru a multi stage charger with the fridge still plugged in no probs.
The only thing I notice is that in hot weather with the fridge running a lot the charger boosts the charge rate a bit to compensate. Had this set up for 2 years & the battery is still holding charge like it was from new.

AnswerID: 508523

Follow Up By: Bushranger1 - Tuesday, Apr 09, 2013 at 08:01

Tuesday, Apr 09, 2013 at 08:01
Just thought I should clarify that I leave the fridge on 12v connected to the battery when the genset is running. I do not switch it to 240v.
FollowupID: 786075

Reply By: ABR - SIDEWINDER - Tuesday, Apr 09, 2013 at 08:09

Tuesday, Apr 09, 2013 at 08:09
Hi Nick

You don't mention the size of the charger but I would recommend at least a 20A charger to make it worthwhile running the generator and charger. Use a clamp meter and check what the total draw is of all the equipment switched on and then buy a charger with a similar capacity.

You can see how to use a clamp meter here: Clamp-Meter

Try not to let the battery voltage drop below 30% on an AGM.


Derek from ABR
AnswerID: 508524

Reply By: Mr Pointyhead - Tuesday, Apr 09, 2013 at 09:21

Tuesday, Apr 09, 2013 at 09:21
Make sure you check the current draw of the fridge you are using. Most of the caters (usually Salvo's) I see on firegrounds usually have fairly large fridges given the amount hungry firefighters eat and drink :). This fridges will pull more than the small 40/50 Litre fridges that many of us run in our 4x4's

AnswerID: 508530

Follow Up By: OzTroopie - Tuesday, Apr 09, 2013 at 23:52

Tuesday, Apr 09, 2013 at 23:52
Thanks Mr Pointyhead, fridge is a 80L Engel, just for small scale catering before the big guns (as you say usually Salvos) get called in with their big fridges and big gensets.
FollowupID: 786156

Reply By: The Bantam - Tuesday, Apr 09, 2013 at 10:49

Tuesday, Apr 09, 2013 at 10:49
In theory it should be fine & beaut to have ya fridge and a charger connected at the same time.
There are thousands of installations been running like this for just about every burgular alarm & telephone exchange and many other installations that absolutely most be no break and run off batteries.

Most of these systems run on a straight forward 13.8 volt regulated charger ( not telephones they run on 50V).....there is very rarely any sort of problem as long as the battery maintenance is done regular.....there are plenty of these systems that run 24/7/365, year after year, for decades.


There have been some problems with SOME multistage chargers getting confused with loads comming and going, and the state of charge rising and falling.

If the programe in the charger has been properly written there should not be a problem...but it does happen.

You would be wise to run the system over a day or two and keep an eye on things.....observe the battery terminal voltage and the display on the charger.

Look out for the charger failing to fall back from the boost charging ( higher voltage mode) or failing to step up from the trickle charging mode...or hunting between modes.

Worst case
Most of the smart chargers have some sort of "supply" or "fixed voltage mode", where the charger puts its brains in its back pocket and works like a fixed regulated supply on 13.5 to 13.8ish volts.
If the charger seems to be getting confused or it is over charging, use this mode.

AnswerID: 508546

Reply By: OzTroopie - Tuesday, Apr 09, 2013 at 23:53

Tuesday, Apr 09, 2013 at 23:53
Thanks all for your replies, most informative.


AnswerID: 508616

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