Split charge relay

[Boldly]

Recently added a split charge relay to my van. Noticed it wasn't working yesterday discovered 40 amp fuse next to cab battery blown. Could this need to be uprated or could something else be wrong. All connections are tight cables not getting warm. New fuse seems ok. Any advice
Thanks
Dennis

 

[autorouter]

What are you using to trigger the relay? Is it the D+ signal from the alternator or the ignition key turn-on? Or is it a voltage-sensitive split charge relay?

 

[Boldly]

Voltage sensitive connected to positive terminal cab battery. When I fitted it LennyHB said fit 40 or 50 amp Fuse. Fitted 40 as I thought that would be safer, maybe needs to be 50?

 

[Boldly]

Not sure how many amps alternator kicks out

 

[autorouter]

Voltage sensitive connected to positive terminal cab battery. When I fitted it LennyHB said fit 40 or 50 amp Fuse. Fitted 40 as I thought that would be safer, maybe needs to be 50?

Worth a try. How many leisure batteries are there? If there's two they will take more from the alternator.

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[Boldly]

There are two when I mentioned it before was 145 amp hours but bought a new(bigger) one in go outdoors sale so now 190 amp hours. Maybe now need bigger fuse

 

[autorouter]

When the split charge relay closes, the alternator sees the starter and leisure batteries as one big battery. If it can send out more amps to raise the voltage to the charging voltage, then that's what it will do.

Another possibility is the arrangement for powering the fridge when the engine is running. Is it a 3-way or compressor fridge? If there isn't a separate fridge relay then the fridge power might be coming from the leisure batteries, possibly adding 10A to 15A to the amps through that wire.

 

[Boldly]

Sorry for the delay in responding. Fridge is domestic 240 volt only ran from ehu or Inverter hence new bigger battery. 200 watt of solar ready to be added. I did run the inverter and fridge whilst driving a couple of weeks ago and is possible that's when the fuse blew as not used leisure battery power since.

 

[autorouter]

Fridge is domestic 240 volt only ran from ehu or Inverter hence new bigger battery. 200 watt of solar ready to be added. I did run the inverter and fridge whilst driving a couple of weeks ago and is possible that's when the fuse blew as not used leisure battery power since.

That's probably what is taking a bit of extra power. If the inverter is running from the leisure batteries, the alternator is trying to charge the leisure batteries and also supply the power for the inverter/fridge. So you probably need a 50A or 60A fuse on the starter battery feed, and the same on the leisure battery feed.

 

[Boldly]

autorouter thanks will put in bigger fuses.
Dennis

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[stevewagner]

Be careful if uprating fuses as you don’t want the fuse capacity greater than the capacity of the cable it’s protecting.

 

[Boldly]

All cable is as supplied with the relay kit. It doesn't state the actual size but it is quite a bit thicker than the cable that comes attached to maxi fuse holder from Halfords. Whilst I know one should never assume anything it seems reasonable that they wouldn't supply a cable too small for the load?

 

[Pausim]

A large battery bank has the potential to suck in more amps than either the wiring or the relay can handle. Hence the fuse blows. Perhaps you need a B2B instead of a split charge relay, this will control the flow and protect the wiring, fuses and alternator.

 

[cmcardle75]

One of the advantages of a B2B is that you know how many amps its going to pull, as it is properly controlled. A split charge relay just lets it make its own mind up. The current could be too small and not charge the battery properly. It could also be too large and blow fuses.

 

[autorouter]

It doesn't state the actual size but it is quite a bit thicker than the cable that comes attached to maxi fuse holder from Halfords.

Often the cable size and spec is stamped on the cable insulation. You can see it more easily if you shine a torch at a shallow angle to show it up.

There's two issues with cable size, heating and voltage drop. The usual recommendation is to keep the voltage drop below 3%. It depends on the cross-sectional area (CSA) in square millimetres (mm2), as well as the length and the amps. A cable that is OK for voltage drop will be well within spec for the heating spec.

If it's a problematic voltage drop you will be able to measure it directly with a meter. You have to measure while the amps are flowing through the cable. 3% of 12V is 0.36V, ie 360mV. Not very high but measurable.

The max amps of the relay should be on the label of the relay.

The voltage drop for a cable, given the length, CSA and amps, is very predictable. There are calculators and charts available, such as this one:

Voltage Drop Calculator | 12 Volt Planet

Use our Voltage Drop Calculator to select the correct Cable for your campervan, motorhome or boat wiring application. Auto and Marine Electrical Components from 12 Volt Planet - Cable Size Calculator

www.12voltplanet.co.uk

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[DBK]

The fuse on the split charge relay on our van was 100 amps, now not used since fitting a B2B.