Second battery charging

[don simon]

I'm happy about second battery install location and had a look at the cable run to second battery and am also happy with that. I now need to buy some more cable and just want to be sure that the altenator which is rated at 70 amp will only need 70 amp cable to run to the second battery.
Yay or nay?

 

[solarman216]

More than enough, it will rarely if ever charge at 70 amps anyway, Rick

 

[solarman216]

But you must not connect your second battery direct to the alternator, when starting your 70 amp cable will fry as it will in effect be connected to the start battery, you need a charge sensing relay, so it is only connected once engine running and charging, Rick

 

[jims-terrano]

Try this little gadget to seperate and control charging.

https://www.jgs4x4.co.uk/t-max-12v-...charging-system-relay-digital-status-monitor/

 

[don simon]

More than enough, it will rarely if ever charge at 70 amps anyway, Rick

That's what I expected, the alternator at below 70amp and cable in reality able to cope with over 70 amp.
Wiring as per the diagram below.
20170727_200820 by Simon Carter, en Flickr
Cheers Rick.

 

[Lazy-Ferret]

While, as Rick says, you are very unlikely to get 70amp from the alternator, if you have run the second battery low, when you start the car, and the split charge relay clicks in, it will effectively connect the discharged battery to the cars own well charged battery. This will try to charge the low battery, until they are at an equal voltage, which can generate huge currents.

Another thing is, always run the biggest wire you can afford, the voltage drops at 12v are surprisingly high, and may result in the second battery never getting fully charged. Go to this website, scroll halfway down, and put in the cable size, current and length of run, and you will be amazed at how much the voltage falls. http://www.12voltplanet.co.uk/cable-sizing-selection.html

For example, if you run 4 meters of 30amp (4mm) cable, to the positive, and another meter from the negative to a local ground point, that is 5 meters of cable. If you now draw the full 30 amp, you will lose 1.28 volts. If the alternator is in tip top condition, you might get 14 volts out, but that means you will only be seeing 12.72 volts at the battery, which is not enough to fully charge it.

On top of that, the fuses and relay will all also cause a voltage drop as well.

 

[don simon]

While, as Rick says, you are very unlikely to get 70amp from the alternator, if you have run the second battery low, when you start the car, and the split charge relay clicks in, it will effectively connect the discharged battery to the cars own well charged battery. This will try to charge the low battery, until they are at an equal voltage, which can generate huge currents.

Isn't this what the fuse is for?

 

[Lazy-Ferret]

Isn't this what the fuse is for?

It is, but you don't want to be blowing the fuse every time you start the car after a nights camping, especially as the first you are likely to know about it, is when you get to camp the next night, and find the battery is dead.

Mine is linked with 200 amp welding wire, has 175amp fuses each end, and a 200amp relay.

 

[jims-terrano]

Surely a voltage sensing unit whether it's a cheap one or like the one I linked is the way to go. It'll start the charge when main battery/alternator are up to a predetermined voltage before switching the second battery into circuit. This protects main battery from discharge and also the second battery is isolated when starting.

 

[don simon]

That's fine, that's what I was trying to get my head around. Where do you stop when preventing problems and where do the problems start in the real world?
If it does blow fuses, It's not that big a problem to upgrade.
I'll want to watch the consumption of the fridge as I think that's going to be a power hungry little bugger. That's my main concern for running the battery down.

 

[jims-terrano]

I had a cheap charge fridge relay for the towbat wiring on the project truck when I got it. I've utilised it to control decent relays for split charge on my second battery and also to feed the 12s socket. It's voltage sensing and seems to do a reasonable job. I ran the battery quite low with a 12v cool box and the charging circuit did a good job of charging the battery again afterwards. I have decided the next time I setup a split charge system I am going to use one of those tmax gadgets.

 

[don simon]

Surely a voltage sensing unit whether it's a cheap one or like the one I linked is the way to go. It'll start the charge when main battery/alternator are up to a predetermined voltage before switching the second battery into circuit. This protects main battery from discharge and also the second battery is isolated when starting.

From my understanding, the question is whether that is really necessary?
From what are we protecting the second battery from when starting?
If I understand Clive, if the second battery is discharged too much, it won't matter when you connect the starter battery to the second battery, there will be a surge causing everything to fry.
The relay (VSR) I have prevents drainage of the main battery when switched off, and pretty much does the protetion job. Or I haven't understood anything...

 

[Lazy-Ferret]

Surely a voltage sensing unit whether it's a cheap one or like the one I linked is the way to go. It'll start the charge when main battery/alternator are up to a predetermined voltage before switching the second battery into circuit. This protects main battery from discharge and also the second battery is isolated when starting.

Voltage sensing is in fact worse, as it means the car battery is at a much higher voltage, when the second battery is switched into the circuit.

That's fine, that's what I was trying to get my head around. Where do you stop when preventing problems and where do the problems start in the real world?
If it does blow fuses, It's not that big a problem to upgrade.
I'll want to watch the consumption of the fridge as I think that's going to be a power hungry little bugger. That's my main concern for running the battery down.

That's what I was thinking, there is a good chance the battery will be very low most mornings after you have used the fridge, and you want the battery as highly charged as possible when you stop for the night.

70 amp is a good start, but what relay are you going to use? You can get the 200amp ones pretty cheap on Ebay, which reduces the chances of that welding closed, should you draw too much current on switch on, and at least with 70amp cable, once the charge current has dropped down to 10 or 20 amp, which is still a good charge rate, the volt drop is less than a quarter of a volt.

 

[Lazy-Ferret]

I had a cheap charge fridge relay for the towbat wiring on the project truck when I got it. I've utilised it to control decent relays for split charge on my second battery and also to feed the 12s socket. It's voltage sensing and seems to do a reasonable job. I ran the battery quite low with a 12v cool box and the charging circuit did a good job of charging the battery again afterwards. I have decided the next time I setup a split charge system I am going to use one of those tmax gadgets.

Most towbar wiring actually utilises the voltdrop in the long cable runs, plus the multiple connections, which because of the relativity higher resistance, stops the huge currents I was talking about, but is also why the caravan battery does not fully charge when you tow it.

I know Don will be pushing his battery to the limits, so needs the best possible charge, in the shortest possible time, but that then creates the issues with higher currents.

 

[don simon]

I know Don will be pushing his battery to the limits, so needs the best possible charge, in the shortest possible time, but that then creates the issues with higher currents.

The fridge isn't going to be running at max power as I use freezer blocks in it, they can keep a good temp on their own for a few days (but it is less controllable). I won't be using this configuration for more than a couple of days and a new fridge is on the horizon.

 

[Lazy-Ferret]

The fridge isn't going to be running at max power as I use freezer blocks in it, they can keep a good temp on their own for a few days (but it is less controllable). I won't be using this configuration for more than a couple of days and a new fridge is on the horizon.

I think you should be OK with the 70amp wire, just make sure you carry some spare fuses. :thumb2

I have one of those average power consumption meters, and I keep meaning to take it with me the next time I use the freezer, and see how much time it actually spends turned on. I wonder if you crank it down to minus 12, if it just basically stays on all the time.

Just to add another thing to the equation, I am not sure what size battery you are going for, but remember if you have an 85A/h battery, and you run it flat, you have to get 85 amps back into it, so if you are driving for 8 hours, that is nearly 11 amps an hour, and even more for a 110A/h, so like I said, in the first instance, a lot of that will come through at a much higher rate initially, one post I read said anywhere up to 300amp can pass between a dead battery and a good battery when you initially connect the Jump leads, which is pretty scary. I think you should be going for circuit breakers, rather than fuses on the install, and make them where you can check them fairly easily. :augie

 

[rayf3262]

If you want your aux battery to last, then as with most LA batteries you should aim for around a maximum 50% discharge, so in reality a 110AH battery will reliably deliver 50~60AH.

 

[solarman216]

If you want your aux battery to last, then as with most LA batteries you should aim for around a maximum 50% discharge, so in reality a 110AH battery will reliably deliver 50~60AH.

Ray is spot on here, in fact if it is a start battery with a leisure label (most are) then 30% discharge is mandatory, my full traction batteries can only go to 80% and even that is not recommended all the time, my first battery is now 12 years old and still producing good results but drinks water, this time of year around 2.5 ltr a week, Rick