How many batteries can I charge?

[Steve H Junior Member Find all posts View Profile Close Steve H]

I have a petrol 120 which has an allrounder main cranking battery, Then through a dual battery isolator, I have a 120a Deep cycle AGM mounted in a drawer system which powers an 80l Waeco. This has been working well for me so far but I have noticed even after driving all day, The rear AGM battery only really gets up to about 12.6-12.8v where as it will go way over 13 when charged by a 240VAC 25a charger. (Cables between batteries are almost welding leads so no issues there) Now I have bought a small van and I was considering running another set of cables from the Prado's rear aux 120a AGM, through an Ando plug to the vans battery which is a 100a AGM. Van has 90l Dometic fridge, led's, pumps etc...My question is Will the alternator be ok to charge all 3 batteries while driving? I have purchased a diode booster hoping it might assist a little and also, is it a problem to have batteries of a different size? Thanks for reading.

[Guest Close Guest]

I would also consider having the 2 AGM batteries plugged in occasionally to a 240VAC multi-stage charger to maintain the health of the batteries.

[Steve H Junior Member Find all posts View Profile Close Steve H]

I would also consider having the 2 AGM batteries plugged in occasionally to a 240VAC multi-stage charger to maintain the health of the batteries.

Thanks mate, yeah I put the 240V Charger on about once a month but do you think the alternator will charge all 3 and is it ok to have different battery sizes all connected? Thanks for your reply.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

What is your charge voltage at the cranking battery terminals with the motor at 1500RPM and with all batteries fully charged?

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi Steve and depending on when you measured the voltage, it sounds like your battery is fully charged.

A fully charged battery, just after the charging source is removed, can have a voltage reading from 12.7v to what ever the charging source was applying, say 13.6v or higher.

This initial voltage reading is seeing the surface charge voltage, something all lead acid batteries have, straight after being charged.

If the battery has a small load on it, like a fridge, it will use up this surface charge fairly quickly and then you will be measuring the battery’s actual voltage.

A fully charged battery, even with a small load, like a compressor fridge, will have a voltage reading of around 12.7v.

The chart below will give an idea of the state of charge of your battery, while it’s in use.

As to how many batteries you can charge, you can actually charge as many batteries as you like.

The more batteries you have, and the lower they are, can slow down how fast you recharge them, but do not try to charge house batteries ( batteries located in a caravan ) and run a 3 way fridge off the same cabling coming from the tow vehicle.

A 3 way fridge’s current draw on any size cabling, will cause so much voltage drop that you will not charge low house batteries properly and in worst cases you can even have a situation where you leave home with full charged house batteries and get to your destination only to find the 3 way fridge has discharged the house batteries while you were driving.

The most effective way to set up a caravan to run a 3 way fridge and charge house batteries at the same time is to run two separate sets of twin power supply cables from the cranker battery to the caravan.

One set of cables will power the fridge and the other set is used to charge the house batteries.

There are other ways to achieve similar results but the two sets of twin cables is the most efficient.

[Trevc Senior Member Find all posts View Profile Close Trevc]

I wonder if there would be a benefit from installing a 2nd isolator and running power to the tug via that.
I'm not sure if there would be any benefit in charging but you essentially have a 2nd circuit from the crank which could be good for other reasons.

Trev

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi Trevc and because all isolators are set to turn on once the alternator is running, the second isolator would still turn on at the same time the first one does.

So unfortunately there is no advantage.

[Steve H Junior Member Find all posts View Profile Close Steve H]

What is your charge voltage at the cranking battery terminals with the motor at 1500RPM and with all batteries fully charged?

I'll check that later on this arv when the car is back. Havnt installed the booster yet but Im assuming it will be somewhere around 13.5?

[Steve H Junior Member Find all posts View Profile Close Steve H]

Hi Steve and depending on when you measured the voltage, it sounds like your battery is fully charged.

A fully charged battery, just after the charging source is removed, can have a voltage reading from 12.7v to what ever the charging source was applying, say 13.6v or higher.

This initial voltage reading is seeing the surface charge voltage, something all lead acid batteries have, straight after being charged.

If the battery has a small load on it, like a fridge, it will use up this surface charge fairly quickly and then you will be measuring the battery’s actual voltage.

A fully charged battery, even with a small load, like a compressor fridge, will have a voltage reading of around 12.7v.

The chart below will give an idea of the state of charge of your battery, while it’s in use.

As to how many batteries you can charge, you can actually charge as many batteries as you like.

The more batteries you have, and the lower they are, can slow down how fast you recharge them, but do not try to charge house batteries ( batteries located in a caravan ) and run a 3 way fridge off the same cabling coming from the tow vehicle.

A 3 way fridge’s current draw on any size cabling, will cause so much voltage drop that you will not charge low house batteries properly and in worst cases you can even have a situation where you leave home with full charged house batteries and get to your destination only to find the 3 way fridge has discharged the house batteries while you were driving.

The most effective way to set up a caravan to run a 3 way fridge and charge house batteries at the same time is to run two separate sets of twin power supply cables from the cranker battery to the caravan.

One set of cables will power the fridge and the other set is used to charge the house batteries.

There are other ways to achieve similar results but the two sets of twin cables is the most efficient.

Thanks Drivesafe. I have read what you said and might need some time to wake up a bit more yet... I have a battery in the van that will be connected to the 3 way fridge in the van and there will be separate cables coming from the aux battery in the car going straight to the van battery just for charging while driving. Does that sound like a correct way to set it up?

[Steve H Junior Member Find all posts View Profile Close Steve H]

Also there will be a 100a solar panel on the roof, connected to the van battery for a little extra help. Sound ok?

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi again Steve, your 3 way fridge should not be connected to your batteries in the caravan.

Another solution to the 3 way fridges high 12v current draw is to not run it on 12v at all.

Simply cool the fridge down the night before you travel, then leave it off while you drive.

When you get to where you are going, switch the fridge on, with either gas or 240 mains power.

I have many customers who do this and they don’t have any problems keep food cold, even in t heat of the far north.

[Steve H Junior Member Find all posts View Profile Close Steve H]

Hi again Steve, your 3 way fridge should not be connected to your batteries in the caravan.

Another solution to the 3 way fridges high 12v current draw is to not run it on 12v at all.

Simply cool the fridge down the night before you travel, then leave it off while you drive.

When you get to where you are going, switch the fridge on, with either gas or 240 mains power.

I have many customers who do this and they don’t have any problems keep food cold, even in t heat of the far north.

Thanks heaps for your info. Yes that makes sense. Just keep the van battery for running lights and pumps and maybe a small telly or radio and leave the van fridge for gas or 240. I have the Waeco in the car which will be running 24/7. Maybe even throw a frozen water bottle in the van fridge to help it stay cold while driving. Does sound a lot easier than worrying about killing batteries by flattening them.

[KA_WA_11 Junior Member Find all posts View Profile Close KA_WA_11]

Drivesafe,

Instead of running a second set of cables couldn't you install a redarc dc to dc 6amp in vehicle charger to achieve the same result? I don't believe there would be much of a difference in price?

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Assuming in van, the absorption fridge with draw around 15A that would would only leave 5 to charge the van battery and having the load connected to the charger is going to upset the mutli stage function. If he is already having a problem with voltage drop, the charger is not going to help it will only be around 70% efficient, and that's at 14.5V input if we drop the voltage down to around 12V then it's going to be drawing a lot more current and unless it is a low voltage model probably won't work. Nothing makes up for under size cabling! Assuming in car with the fridge load your still going to get considerable voltage drop, it will help assuming the output voltage of the charger is higher than that of the alternator, it might not be though. Again nothing makes up for under size cable.

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

Hi again Steve, and how much power are you contemplating to need.

Are you looking at stopping for night, using a bit of battery power and then driving to another stopover?

Or are you likely to be free camping for a few days between drives?