Linking Batteries

[TiggerToo]

.. still musing about batteries, and mm-ing and ar-ing about the joys of Li vs Pb.

In common with many other sailing Tigger has the possibility of "linking up" the two banks (at least temporarily) with a "both" key.

While I realise that it is not a good idea to have Li and Pb batteries permanently linked without some sort of B2B charger, would a temporary connection (e.g. in an emergency) result in some bad damage? (If so, I can "simply" remove/cover the "link" option.)

 

[Tranona]

Check whether the "both" is paralleling or whether it is linking the house bank to the engine start circuit to start the engine. Assume you have a "conventional" 2 separate independent banks, one for engine start only and the other for services charged through some form of split charging system.

 

[William_H]

On typical boats the link is primarily to provide a "jump" start to a tired engine battery. With all lead acid when the engine starts and charges the link provides charging for both batteries. Ideally you would have a diode or VSR to provide this charge of house battery in normal operation.
Now if you have a Li house battery you could set up the link switch so li battery can provide additional current to engine battery for that jump start. But then you would worry having the Li battery connected to alternator and engine battery even for a short period of charging of Li.
So I can see OP question but don't have an answer as this must have come up before. I would think in terms of a large diode (silicon) in series with link switch so Li battery can provide some amps to boost engine start battery but would not permit any charge to Li. It's charge being done by proper charge control. ol'will

 

[TiggerToo]

Assume you have a "conventional" 2 separate independent banks, one for engine start only and the other for services charged through some form of split charging system.

you assume correctly

 

[ibbo]

If the voltages are close then it could be done. I have Li, and Lead permanently connected. Of course if your battery is flat, then the voltages will be way out.
The danger is the Li will dump loads of amps into a depleted lead battery.
As long as the Li aren't near to full charge you could start on Li alone, but I don't see how you could safely swap to your start battery. You would need to stop the engine before the batteries shut off at high charge.

Still leaving a flat battery on engine start.

You could quickly connect the start, and disconnect the Li, and hope.

Mike

 

[Tranona]

you assume correctly

Then it gets back to the question as to whether the linking switch parallels the batteries or just connects the house bank to the engine circuit. If the latter not a problem but if the former then it is as ibbo says.