Weird short circuit

[Na Mara]

I have just wired up my LFP bank. At present its only connections to the boat are on the negative side to my multiplus 2 inverter charger and through that to the common negative via the earth for that unit, and to the 12DC for the boat on the positive and negative. So 3 connections to the boat 12V in total. There are literally no other connection between the LFP bank and the rest of the boat except to the AC 230V system through the inverter.

On the positive side between the bank and the 12DC system there is a battery protector that is refusing to connect the LFP bank due to the presence of a short circuit. Sure enough, upstream of the main 12V DC breaker but downstream of the protector there is zero resistance between the positive of the LFP bank and the positive of the engine battery. These should be entirely disconnected from each other (infinite resistence).

Currently the battery protector is doing its job and so no damage is being done but its annoying not being able to run the 12V off the Lithium as intended. I just can't see how these banks can be connected except through the common negative earthing to the engine block. But why on earth (excuse the pun) would that generate a short between the two batteries?

I'm getting to the point of just disconnecting the AGM battery entirely and going all lithium to solve the problem. That should solve it, but at the loss of an AGM battery for anything other than emergency backup. Anyone have any (brighter) ideas what the short could be?

[donradcliffe]

Multimeters don't measure the resistance between two positive voltage sources.

Watch this video.

https://www.youtube.com/watch?v=lRAsJOisX2g

I would use a resistor to limit the inrush current while the caps in the inverter are charging.

[Na Mara]

Thanks for the reply but now I’m even more mystified. The short circuit is not on the LFP battery side of the protector. The External BMS is not cutting out the bank and the breaker on the positive of the inverter is open so it’s not connected to the bank on the positive side ( so no capacitance issue, though I will bare that one in mind in case of future issues). I can see the state of the LFP batteries and they are stable and chipper, they are not discharging.

The short has to be downstream of the battery protector, otherwise the BMS would intervene and not the battery protector, or there would be some discharge.

If it’s not a short between the banks, then I really have no idea where to look for the short.

[donradcliffe]

We may have a language problem. Is this what you mean by a battery protector?

https://www.victronenergy.com/batter...attery-protect

[smac999]

draw a wiring picture... and provide actually model of this "battery protector"

[Na Mara]

The battery protector is the 220A version of the Victron battery protector.

It shows the correct battery voltage for the LFP bank but displays the error code for a short circuit and thus disconnects the LFP bank from the 12DC system of the boat.

I’ll get a wiring diagram up in a bit but I’m not on the right computer for that at the moment so it will take a minute. However, I promise that the only connections presently between the LFP bank and the boat are:

1. through the negative side through the invertercharger.
2. Through the negative side to the common earthed negative for the boat.
3. Through the positive side to the 12DC system breaker.

The short persists even with the 12Vdc breaker open, so the short appears to be between the 12VDC breaker and the battery protector. But blow me If I can find it.

How do you measure if there is a short between two battery banks if you can’t do it using a multimeter?

[Recy]

I think this is what happens.

https://www.youtube.com/watch?v=ZlrtmJRfSP8

[Na Mara]

Again, I cannot see how it could be a capacitance problem with the inverter for three reasons.

1. The breaker between the positive of the LFP bank and the inverter is open. The inverter is not connected to the positive of the batteries.

2. The BMS I have is the top of the range victron Lynx 500 BMS that apparently has an inbuilt current surge inhibitor. It is specified to be used with high output inverter chargers without pre-charging circuits (see all the very many wiring diagrams for such systems).

3. The BMS also has an inbuilt contact relay on the positive bus as a last defence to protect the battery bank, if the short was upstream of the battery protector, which it would be if it was something to do with the inverte/?charger the BMS would be opening the contact relay to protect the LFP batteries. This it is not doing.

Here is the wiring diagram as promised. The full system is not yet installed and there has been a little deviation from the diagram but not with respect to how the inverter, main distribution system and battery protector is wired up. The main difference is that at the Main distribution panel (far left), the LFP bank is only connected to the 12 DC system and not to anything else. The windlass, starter, sail systems etc now all get their juice from the AGM starter battery not the LFP bank. There is also a wire shown in the diagram called "LFP bypass", this connection is not normally in place. It is only used in the event of the LFP bank being offline so the boat can be run on the AGM battery alone. Unfortunately the diagram needs to be downloaded to be read properly. Its format is a bit strange because the right side of it was to scale in order design the layout of the system as installed.

[smac999]

the drawing is too small to read. (even when opened..)

is the output of the "battery protect" (not battery protector) getting 12v from the agm? the battery protects are only one direction so it may not like that.