320ah Liesure battery bank. Split charge relay in a camper van.

[Handmade Matt]

Hi folks.
I have a camper van that I'm building. It has 390 watt PV on the roof. An MPPT controller and a battery bank of 320ah.

I am connecting up a split charge relay for charging this battery bank from the vehicles alternator when driving. It's positioned under the passenger bench seat in the cab close to the engine. It's a 2.9 Turbo Diesel Mercedes Sprinter 2003 and I believe the alternator to be rated at 90amps

The way I'm doing this is connecting a 100amp relay into a heavy duty cable between the positive terminal of the leisure battery bank and the positive terminal of the vehicles battery, a run of about about six foot. Both batteries are earthed to the vehicle chassis negative. This cable will be fused at either end with 150amp mega fuses. The relay is then triggered from a thin wire that was directly coming from the alternator. This only goes live when the alternator is running, not when just the ignition is switched on.

My concerns/ questions are:

1) If the leisure battery bank charge is very low (although I intend to avoid this anyway.) The system will potentially result in a very high current when the engine starts. Do I need to limit this or will it be OK? (Possibility: I am going to be installing a shunt on the negative earth strap of the leisure battery bank for an ammeter, will this limit the current to it's rating? (Say a 100amp shunt.))

2) Will the alternator be OK if I don't limit the charge rate?

2) Would the split charge cable be better off connected directly to the alternator instead of the vehicle battery positive terminal?

Additionally:
I intend to fit a momentary sprung push switch that you have to hold down. This will trigger the relay to allow a reverse flow to the vehicle battery if it is ever flat. Basically an emergency override to essentially jump start the camper. I'd hold the button down for about thirty seconds, preferably with a sun full of PV and then attempt to start the engine. I understand that this is not good for my leisure batteries but it would be in an emergency.
Does anyone have any comments on this?

Thanks.

[Sunking]

Stop what you are doing right now before you burn the thing up. You do not know what you are doing. YOU DO NOT WANT TO USE A RELAY.

Understand?

You want to use an Electronic Battery Isolator. They come in all shapes and sizes for $100 and less. The minimum configuration is 1 input port from the Alternator and 2 output ports, 1 for the SLI battery in your vehicle, and another to the AUX battery you are adding. They come in from 50 to 200 amps.

[Handmade Matt]

Of course I understand your insturction when you write it like that, what I don't understand is why.

If anyone would like to help me learn why this is a problem I would be keen to know. I can't see how this could ever cause a fire if it's fused.

People use relays for split charging all the time and there are many wiring diagrams online detailing exactly my plan. I have done it before on other vehicles and I have used voltage sensing relays.
Is it simply different because my leisure battery bank is of such a high capacity? Could I not just use a resistor to limit the current?

(I'm not arguing, I'm trying to learn.)

An electronic Battery Isolator seems very expensive in the UK. I can't find many available and they are marketed at the racing car market. There is one I have found rated at 90 amps that is £67 which isn't so bad. Most are around £200.

[Sunking]

It is real simple your starting battery requires a slightly higher charging voltage than you Hybrid batteries. In addition your Hybrid batteries cannot take the high charge rates that will be provided by the alternator. Between the higher voltages and high current available will greatly reduce the Hybrid battery life. The designs you are seeing are antiquated before electronic isolators were available. The new isolators allow you to set the voltage appropriate for both the SLI and Aux battery.

As for pricing I do not know what is going on over there other than you are being ripped off. Here on this side of the pond less than $100 USD. Good luck

[Mike90250]

a simple relay and resistor cannot provide the proper charge current. And even a fuse, is a resistor. (small, but a resistor, which is why they heat up and blow, sealed in their container)

Most times, the store bought relays have enough internal resistance with their lousy contacts, they mostly limit the current till their contacts fry. When one battery is appreciably lower in charge than the other, and that relay kicks in, the fun can start.

Smartguage in the UK makes some good products, but they are pricey.

http://www.smartgauge.co.uk/products.html

and here's the breakdown of the internal differences of cheap vs real relays:
http://www.smartgauge.co.uk/more_relay.html read all the way down to the end photo too.

""Yet the price of the far eastern one is typically around £15.00 in the UK. Our 200 amp relay (not in the photograph) is twice this price. But if our 100 amp relay is so much better imagine what the 200 amp unit is like. ""

[Handmade Matt]

Thanks for the reply.
I've been unable to find a product where it's possible to set charging voltages.

These are the two most suitable candidates so far. What do you think? Which would you choose?

http://www.ebay.co.uk/itm/Stinger-S9523-Battery-Isolator-/370747513550?pt=UK_In_Car_Technology&hash=item5652461ace

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=151181682608&ssPageName=ADME:X:RTQ:GB:1123

[Handmade Matt]

I've since spoken to quite a few people and read a whole bunch online about doing this the "antiquated way."
I think it's the direction I'm going to go.

A 200amp relay triggered from the alternator. The wiring that I have is very thick, like jumper cables with heavy duty fuses
I am considering fitting a small 12v DC delay timer to connect in the relay after 5 minutes of alternator time to recharge the vehicle battery from the start.

Thanks for all of the advice though.