‘Fessing up. My dubious LiFePo install.

[jakew009]

A fully charged LA battery is going to have a resting voltage of about 12.8V, a lithium battery will be at around that at 20% SOC, so unless the Lithium battery is totally flat, no current can flow. Plus, the BMS should stop the battery being totally flattened.

It could get ugly if it’s the other way round though, dead lead acid being “charged” by 100% lifepo4.

 

[PaulRainbow]

It could get ugly if it’s the other way round though, dead lead acid being “charged” by 100% lifepo4.

What do you think would happen ?

 

[jakew009]

What do you think would happen ?

You’ve posted that picture of the exploded lead acid battery enough times before.

I’m pretty sure if you connected a lead acid battery with a shorted / failed cell in parallel with an LFP the result would be ugly.

I agree with you that in principle connecting LFP and lead acid in parallel works (heck there are thousands of people out there doing exactly blissfully unaware).

However, it’s what happens when things go wrong that is the problem. But you already know that

 

[PaulRainbow]

You’ve posted that picture of the exploded lead acid battery enough times before.

I’m pretty sure if you connected a lead acid battery with a shorted / failed cell in parallel with an LFP the result would be ugly.

If the cell was shorted the result could be the same as if it was on charge with the shore charger. It may indeed explode, but no difference to any other shorted LA battery. On the other hand, it could be that the current passing from the Lithium to the LA would blow the fuse. Testing this isn't on my ToDo list though

I agree with you that in principle connecting LFP and lead acid in parallel works (heck there are thousands of people out there doing exactly blissfully unaware).

However, it’s what happens when things go wrong that is the problem. But you already know that

It does work, but even then it isn't perfect, as my charging currents prove. A little more experimenting with mine as it is, then adding the DC-DC chargers.

That will leave some work with deciding the best way to start the engines in the event of an engine battery failure. Easy with small yacht engines, they can often be started from the Lithium battery in and emergency, or a Lithium jump pack. Not so easy with 7 litre engines and above.

 

[jakew009]

If the cell was shorted the result could be the same as if it was on charge with the shore charger. It may indeed explode, but no difference to any other shorted LA battery. On the other hand, it could be that the current passing from the Lithium to the LA would blow the fuse. Testing this isn't on my ToDo list though

Indeed, but most shore power chargers wouldn’t be able to supply thousands of amps back to hoping the fuses work.

It does work, but even then it isn't perfect, as my charging currents prove. A little more experimenting with mine as it is, then adding the DC-DC chargers.

That will leave some work with deciding the best way to start the engines in the event of an engine battery failure. Easy with small yacht engines, they can often be started from the Lithium battery in and emergency, or a Lithium jump pack. Not so easy with 7 litre engines and above.

That why I fitted a shunt on my starter battery - so I can measure how much power the starter motor actually uses.

I suspect it’s considerably less than 500A in which case my LFP batteries will easily cope with it in an emergency.

We have one of these for the trucks, it will start just about anything but not cheap.

NOCO GB251+ Boost MAX 3000A 24V UltraSafe Lithium Jump Starter with Power Bank

 

[PaulRainbow]

Indeed, but most shore power chargers wouldn’t be able to supply thousands of amps back to hoping the fuses work.

But that's the point, a shore charger might be 30A, which is enough to keep putting 14+ volts into the battery, boiling it and making it gas, then a spark could ignite the gas. If in parallel with Lithium it's more likely that the current would be excessive and I very much doubt doubt the 200A fuse would withstand thousands of amps.

That why I fitted a shunt on my starter battery - so I can measure how much power the starter motor actually uses.

I suspect it’s considerably less than 500A in which case my LFP batteries will easily cope with it in an emergency.

We have one of these for the trucks, it will start just about anything but not cheap.

NOCO GB251+ Boost MAX 3000A 24V UltraSafe Lithium Jump Starter with Power Bank

My starter motors are 5.4Kw @ 24V so i can't start them from the Lithium battery as the BMS is only 200A. I have some 300A BMS's so might try one of those. I could also fit a second set of engine batteries, so each engine has it's own batteries, with an emergency switch. Or, i could fit a second set of Lithium batteries, also with a 300A BMS.

Couldn't justify a jump pack at that price, another set of Lithium cells and a BMS would be less than half of that.

 

[geem]

But that's the point, a shore charger might be 30A, which is enough to keep putting 14+ volts into the battery, boiling it and making it gas, then a spark could ignite the gas. If in parallel with Lithium it's more likely that the current would be excessive and I very much doubt doubt the 200A fuse would withstand thousands of amps.

My starter motors are 5.4Kw @ 24V so i can't start them from the Lithium battery as the BMS is only 200A. I have some 300A BMS's so might try one of those. I could also fit a second set of engine batteries, so each engine has it's own batteries, with an emergency switch. Or, i could fit a second set of Lithium batteries, also with a 300A BMS.

Couldn't justify a jump pack at that price, another set of Lithium cells and a BMS would be less than half of that.

You could fit a bypass circuit on the BMS to allow the two lithium batteries to start the engines in parallel?
My boat is also 24v. I have a pair of 100Ah starter batteries in series for starting. My diesel genset is 12v so I always thought of would just disconnect a faulty engine battery and connect the genset battery. Not perfect but should work. The lead batteries are all in the same battery box so it would be pretty simple to swap wires about rather than moving batteries

 

[geem]

But that's the point, a shore charger might be 30A, which is enough to keep putting 14+ volts into the battery, boiling it and making it gas, then a spark could ignite the gas. If in parallel with Lithium it's more likely that the current would be excessive and I very much doubt doubt the 200A fuse would withstand thousands of amps.

My starter motors are 5.4Kw @ 24V so i can't start them from the Lithium battery as the BMS is only 200A. I have some 300A BMS's so might try one of those. I could also fit a second set of engine batteries, so each engine has it's own batteries, with an emergency switch. Or, i could fit a second set of Lithium batteries, also with a 300A BMS.

Couldn't justify a jump pack at that price, another set of Lithium cells and a BMS would be less than half of that.

Interesting about your starters. My starter is a Prestolote S115 24v quoted at 7.8kw. My engine is only 4.4L but seems to have a monster of a starter

 

[PaulRainbow]

You could fit a bypass circuit on the BMS to allow the two lithium batteries to start the engines in parallel?
My boat is also 24v. I have a pair of 100Ah starter batteries in series for starting. My diesel genset is 12v so I always thought of would just disconnect a faulty engine battery and connect the genset battery. Not perfect but should work. The lead batteries are all in the same battery box so it would be pretty simple to swap wires about rather than moving batteries

Yes, did think of a bypass switch. I already have an emergency parallel switch for the positive, from when the LA were fitted. Not sure how well my 280Ah pack will start the engines, need to see the current draw with the inrush at startup. If i have to fit another Lithium pack i think the engines might start without the bypass.