Negative/positive

[MetalMonkey]

Think of it like this - the fuse is to protect the cable.

If the second battery is next to the first and you are just adding a couple of very short link cables - I wouldn't bother with a fuse.
If the second battery is a distance away and the cable has to pass through bodywork etc to get to it, then fuse the cable to avoid the chance of it shorting out.

The negatives on both batteries should be connected to the chassis (assuming a negative earth vehicle), so therefore don't need protecting against shorting to the chassis....

Obviously any device connected to either battery should also be protected by a suitable fuse.

 

[pedrobedro]

The negatives on both batteries should be connected to the chassis, so therefore don't need protecting against shorting to the chassis....

The negatives should be connected together with only one connection to the chassis.

 

[m_c]

@julianf is correct in that a main battery protection fuse doesn't have to be on the positive cable.
The main battery fuse's sole purpose is to protect things in the event of a major short involving full battery current.
The previous generation of Ford Transits had the main battery fuse as part of the earth lead.

Circuit protection fuses, however should always be on the non-grounded/earthed circuit.

 

[Matrixx]

the fuse in A will protect all below, the fuse in B will not blow and the cable will heat up until it melts and sets fire to your vehicle.

 

[MetalMonkey]

Does this help clarify?

Option 1 if both are next to each other, option 2 if you are passing the cable through bodywork or any distance.
Agreed grommets should be used anywhere a cable passes through a panel, irrespective of if it is fused or not.

 

[RonA]

If this is a leisure battery, have you got it on the same charging circuit as the starter battery ?

If so watch out for them both discharging.

When I’ve been sailing it’s standard to have an isolation switch between the two batteries & as soon as the standby diesel is stopped, the isolation switch is opened to ensure you always have power to restart the engine.
I know you’re involved with boats so you’ll know that already.
RonA

 

[nickk]

So,is there a fuse each side of the bulkhead?
I’m using 2 core flex,the sort of size you’d have on a 4.5” angle grinder and assumed as it’s got an outer sheath,didn’t need grommets,am I right or wrong?

 

[brightspark]

Os is there a fuse each side of the bulkhead?
I’m using 2 core flex,the sort of size you’d have on a 4.5” angle grinder and assumed as it’s got an outer sheath,didn’t need grommets,am I right or wrong?

use grommets even if its double insulated

 

[Barking Mat]

When I’ve been sailing it’s standard to have an isolation switch between the two batteries & as soon as the standby diesel is stopped, the isolation switch is opened to ensure you always have power to restart the engine.
I know you’re involved with boats so you’ll know that already.
RonA

We use 'repartiteur de charge', basically a diode box thing, allowing current to flow to multiple batteries, but not be shared when one is pulled upon. Can't think of the name in English.

Battery isolators, charge splitters ?

 

[Moose McAlpine]

We use 'repartiteur de charge', basically a diode box thing, allowing current to flow to multiple batteries, but not be shared when one is pulled upon. Can't think of the name in English.

View attachment 347874

Battery isolators, charge splitters ?

Split charge relay.

 

[Barking Mat]

Split charge relay.

Yeah, but not relays, more diodes. Anyway I'm digressing on this thread.

 

[sako243]

We use 'repartiteur de charge', basically a diode box thing, allowing current to flow to multiple batteries, but not be shared when one is pulled upon. Can't think of the name in English.

View attachment 347874

Battery isolators, charge splitters ?

Split charge relay.

Not what @Barking Mat has shown above - that's an "ideal" diode.

Diodes have an inherent voltage drop (usually about 0.7V) although it can be different, which isn't great because you can never reach full charge readily. The ideal diode is basically two back-to-back MOSFETs with some basic control electronics that switch them on-and-off appropriately. When the MOSFET is switched hard-on then the voltage drop across it can be very minimal (sub milliohm resistance) and much more efficient than a diode. You have to have two back-to-back diodes because the MOSFETs have an intrinsic "body" diode which means that they will conduct in one direction so you attach two back-to-back and turn them on-and-off simultaneously to open up the current path.

 

[Barking Mat]

Here's how we do it on boats.

Not what @Barking Mat has shown above - that's an "ideal" diode.

Diodes have an inherent voltage drop (usually about 0.7V) although it can be different, which isn't great because you can never reach full charge readily. The ideal diode is basically two back-to-back MOSFETs with some basic control electronics that switch them on-and-off appropriately. When the MOSFET is switched hard-on then the voltage drop across it can be very minimal (sub milliohm resistance) and much more efficient than a diode. You have to have two back-to-back diodes because the MOSFETs have an intrinsic "body" diode which means that they will conduct in one direction so you attach two back-to-back and turn them on-and-off simultaneously to open up the current path.

Yes, valid point, electrical newer ones don't have thé voltage drop.

@RonA , yes but clients, would never remember to do that.

 

[Moose McAlpine]

Not what @Barking Mat has shown above - that's an "ideal" diode.

Is it for the same purpose as a split charge relay, with a different mode of operation, or a totally different device for a different purpose? Never head of one before, but familiar with split charge systems.

 

[Barking Mat]

Is it for the same purpose as a split charge relay, with a different mode of operation, or a totally different device for a different purpose? Never head of one before, but familiar with split charge systems.

Same, same I'd imagine.

 

[sako243]

Many purposes for ideal diodes, they're just a switch after all. The benefit they have over relays is speed of operation and no moving parts as everything is solid state.

They also require next to no power to switch as the gate resistance is very high. Relays typically require more and more power the bigger they get to throw the switch.

Trade offs in all situations but relays will offer next to no resistance in the high current path whereas MOSFETs require no very little power to switch.

 

[blackolive (Ste)]

Ive just got one of these. Isolates the leisure battery when the voltage drops (ie engine/alternator not running) unlike a traditional split charge relay it doesnt require a feed from the alternator https://www.ebay.co.uk/itm/Maypole-...2349624.m46890.l49286&mkrid=710-127635-2958-0

 

[m_c]

Is it for the same purpose as a split charge relay, with a different mode of operation, or a totally different device for a different purpose? Never head of one before, but familiar with split charge systems.

It's a split charge diode.
It's essentially two large diodes, but as @sako243 mentions, you never get full charge from a standard alternator due to the voltage drop.

We had a fleet with them fitted, and they ran external alternator regulators to compensate for the voltage drop.
It was a good system when it was working, and the best option at the time it was developed, but it killed alternators regularly, and modern split charge systems are far more reliable.

 

[sako243]

It's a split charge diode.
It's essentially two large diodes, but as @sako243 mentions, you never get full charge from a standard alternator due to the voltage drop.

It's not a diode. If you Google the part number it mentions MOSFETs, at the cost of slightly higher complexity you don't get the fixed voltage drop across the device that you do with a diode. But the MOSFETs have a small resistance when on so you will suffer some small power losses depending on how efficient they are.

 

[m_c]

It's not a diode. If you Google the part number it mentions MOSFETs, at the cost of slightly higher complexity you don't get the fixed voltage drop across the device that you do with a diode. But the MOSFETs have a small resistance when on so you will suffer some small power losses depending on how efficient they are.

That's a newer style one.
You might find it's just a couple relays in the box.
I know some of the systems I've worked on, that were advertised as 'electronic', still made rather loud clicking sounds