LiFePo4 safety advice question

[jessie]

I’m looking to replace my gel batteries (3 x 110Ah) and considering self heating LiFePo. All the things I’ve heard/ read have made me wary for safety reasons. 
I have a dc to dc charger, which can be set to work with LiFePo. My solar mppt charger can also be set to work with LiFePo.

would i need any other equipment or can I safely just replace the 3 gel for 3 LiFePo and reset the chargers?

 

[Alan de Enfield]

7 minutes ago, jessie said:

I’m looking to replace my gel batteries (3 x 110Ah) and considering self heating LiFePo. All the things I’ve heard/ read have made me wary for safety reasons. 
I have a dc to dc charger, which can be set to work with LiFePo. My solar mppt charger can also be set to work with LiFePo.

would i need any other equipment or can I safely just replace the 3 gel for 3 LiFePo and reset the chargers?

 

 

Do you ever cruise or use your engine / alternator to charge the batteries ?

[jessie]

Yes, that is quite important in winter as I’m a cc’er

[Tony Brooks]

Make sure the LFPs have BMSs or if not an external one is vital.

 

Your alternator is at risk from two things with Li batteries.

When the BMS shuts down charging, it can produce a voltage surge in and from the alternator that might cause damage, usually to the alternator.

Most alternators are designed to charge LA batteries so running at high outputs for hours on end, into Li batteries, can burn the alternator out.

 

You imply that you may be aware of this (talk of a DC to DC charger), but you do explicitly say that the alternator will charge an LA battery and then the DC to DC will charge the Li. If you do not do it this way, then you will need more sophisticated kit to control an alternator connected directly to a Li bank.

Edited December 4, 2025 by Tony Brooks

[Jen-in-Wellies]

Since LiFePO4 batts can be discharged deeper without damage, you don't need as much overall capacity. I replaced four 110AHr Lead acids, with a single 310Ahr Lithium. Kept one of the lead acids to use just as a load on the alternator and charged the Lithium via the B2B, as @Tony Brooks describes.

[IanD]

Just now, Jen-in-Wellies said:

Since LiFePO4 batts can be discharged deeper without damage, you don't need as much overall capacity. I replaced four 110AHr Lead acids, with a single 310Ahr Lithium. Kept one of the lead acids to use as a load on the alternator, as @Tony Brooks 

With a B2B in between, I hope? 😉

[Tony Brooks]

Just now, Jen-in-Wellies said:

Since LiFePO4 batts can be discharged deeper without damage, you don't need as much overall capacity. I replaced four 110AHr Lead acids, with a single 310Ahr Lithium. Kept one of the lead acids to use as a load on the alternator, as @Tony Brooks 

 

That makes keeping two alternators simpler on a twin alternator boat, buy my preference would be to disconnect the engine battery alternator and move the domestic alternator charging lead to the start battery. I think it simplifies things and leaves you with an emergency alternator in case the other one fails.

[jessie]

Good point, the dc to dc charger charges the current gel batteries from the starter (LA) and the alternator charges the starter battery.

I’m unsure what the BMS does, could you elaborate on that please?

[Jen-in-Wellies]

5 minutes ago, IanD said:

With a B2B in between, I hope? 😉

Accidentally hit Submit Reply. See edited post. 

3 minutes ago, jessie said:

Good point, the dc to dc charger charges the current gel batteries from the starter (LA) and the alternator charges the starter battery.

I’m unsure what the BMS does, could you elaborate on that please?

Do you have just a single alternator on the engine and if so, what is it's rated peak output? 

6 minutes ago, Tony Brooks said:

 

That makes keeping two alternators simpler on a twin alternator boat, buy my preference would be to disconnect the engine battery alternator and move the domestic alternator charging lead to the start battery. I think it simplifies things and leaves you with an emergency alternator in case the other one fails.

Or do what I did and have two B2Bs. One to charge from the engine alternator and battery, the other to charge from the house alternator and load battery. 80A total charging capacity on mine and redundancy for alternator failure too. 

[nicknorman]

9 minutes ago, jessie said:

Good point, the dc to dc charger charges the current gel batteries from the starter (LA) and the alternator charges the starter battery.

I’m unsure what the BMS does, could you elaborate on that please?

If you buy a Li battery (collection of cells in 1 case) then it will have a BMS - Battery Management System - built in. The BMS monitors cell voltages, temperature, current and can isolate the battery if any of these parameters reaches values that can damage the cells.

 

If you buy individual cells then you also need to buy a BMS to go with them, for protection.

 

Since you are talking about “self heating” I would imagine you are considering something like a Fogstar Drift, which has built in BMS.
he configuration you have, you should be able to swap the gel battery to a Li battery with no problem, provided you adjust the charging parameters appropriately.
 

I would get some kind of battery state of charge monitor such as a BMV712 to keep track of the State of Charge (SoC) of the Li since unlike lead acid, it is hard to determine the charge state by just looking at the voltage. It might be that the Battery has Bluetooth and you can use an App to see the SoC which would be an adequate substitute for an external SoC monitor.

Edited December 4, 2025 by nicknorman

[Tony Brooks]

7 minutes ago, jessie said:

Good point, the dc to dc charger charges the current gel batteries from the starter (LA) and the alternator charges the starter battery.

I’m unsure what the BMS does, could you elaborate on that please?

 

The BMS should:

Disconnect charging when the battery is more or less fully charged.

Disconnect the load when it is more or less effectively flat.

Prevent discharge and charging when the battery temperature is too high.

Ditto too low.

 

As you talk about battery heating, then the BMS should also turn that on and off as required.

 

I am not sure if they will turn the charge off if the voltage gets too high.

[nicknorman]

2 minutes ago, Tony Brooks said:

.

I am not sure if they will turn the charge off if the voltage gets too high.

Yes, should do.

[Jen-in-Wellies]

1 minute ago, Tony Brooks said:

 

I am not sure if they will turn the charge off if the voltage gets too high

The Fogstar BMS will do that if any cell exceeds a set voltage. Disconnects charging, allows discharging. The opposite if a cell voltage drops too low. Allows charging, won't allow discharging. 

[IanD]

2 minutes ago, nicknorman said:

Yes, should do.

Just to be clear -- it disconnects the battery from the charging source (and loads), it doesn't turn the charge source off and leave the battery connected to the load which would be far more useful -- for this you need a system (like mine -- or yours?) where the BMS actually controls the charging sources (MPPT, shoreline, generator).

[jessie]

15 minutes ago, Jen-in-Wellies said:

 

Do you have just a single alternator on the engine and if so, what is it's rated peak output? 

Yes, single alternator, which I think is 50A, that is from looking up the engine though so could be higher. It’s the original alternator on a vetus M4.15

[IanD]

14 minutes ago, Jen-in-Wellies said:

The Fogstar BMS will do that if any cell exceeds a set voltage. Disconnects charging, allows discharging. The opposite if a cell voltage drops too low. Allows charging, won't allow discharging. 

How can it disconnect charging and allow discharging when the battery (with internal BMS) only has one set of terminals? Either the cells are connected to the terminals by the MOSFETs (charging and discharging allowed) or they're not...

 

The BMS will use different conditions (temperature, voltage, current) for charging and discharging to decide when to do this, but it can only do one thing -- connect cells to the terminals or disconnect them. If it's done a HV disconnect to prevent further charging, the battery is disconnected from the load and can't supply power to it, and if it's done a LV disconnect to prevent further discharging (so the load voltage drops to zero) it can't do any recharging (or supply the load) because it's now disconnected -- until any external charge sources (e.g. MPPT) can provide enough power to bring the load voltage back up past the LV threshold.

 

If you mean it doesn't allow *further* charging (or discharging) and disconnects the cells depending on the conditions, that would be correct -- but it's not what you said... 😉 

 

I'm not just being picky here, it's an important difference between using drop-in batteries with internal BMS (like Fogstar) and a system where the BMS controls charging sources -- which *can* do what you say because it has separate control over charging (sources) and discharging (loads).

Edited December 4, 2025 by IanD

[Gybe Ho]

15 minutes ago, jessie said:

Good point, the dc to dc charger charges the current gel batteries from the starter (LA) and the alternator charges the starter battery.

I’m unsure what the BMS does, could you elaborate on that please?

 

The BMS is the silicon chip brain inside a lithium battery, its main job is to prevent your solar and engine alternator from over charging the battery and in a very extreme case it will prevent a battery from melting.

 

You can think of the BMS as a castle wall,  when the battery is fully charged the BMS pulls up the drawbridge to prevent any more electricity getting into the battery no matter how hard the solar and alternator try to push electricity into the battery.

 

The BMS can perform a few more tricks, if a battery gets close to freezing it will prevent charging but still allow electricity out of the battery to power the boat. A BMS can also prevent too much electricity from rushing out of the battery in a split second during an accident like dropping a metal tool across the +ve and -ve connections on the battery.

[Jen-in-Wellies]

6 minutes ago, jessie said:

Yes, single alternator, which I think is 50A, that is from looking up the engine though so could be higher. It’s the original alternator on a vetus M4.15

Ok, the change should be pretty simple. Replace the gel batts with the LiFePO4 and change the B2B and solar controller settings to their Lithium algorithm. What B2B and MPPT controllers have you got? Make and model. It is usually recommended to limit rhe B2B charge current to around half the maximum alternator current to reduce the risk of the alternator overheating and getting damaged.

[jessie]

23 minutes ago, nicknorman said:

If you buy a Li battery (collection of cells in 1 case) then it will have a BMS - Battery Management System - built in. The BMS monitors cell voltages, temperature, current and can isolate the battery if any of these parameters reaches values that can damage the cells.

 

If you buy individual cells then you also need to buy a BMS to go with them, for protection.

 

Since you are talking about “self heating” I would imagine you are considering something like a Fogstar Drift, which has built in BMS.
he configuration you have, you should be able to swap the gel battery to a Li battery with no problem, provided you adjust the charging parameters appropriately.
 

I would get some kind of battery state of charge monitor such as a BMV712 to keep track of the State of Charge (SoC) of the Li since unlike lead acid, it is hard to determine the charge state by just looking at the voltage. It might be that the Battery has Bluetooth and you can use an App to see the SoC which would be an adequate substitute for an external SoC monitor.

Thanks that is helpful info

21 minutes ago, Tony Brooks said:

 

The BMS should:

Disconnect charging when the battery is more or less fully charged.

Disconnect the load when it is more or less effectively flat.

Prevent discharge and charging when the battery temperature is too high.

Ditto too low.

 

As you talk about battery heating, then the BMS should also turn that on and off as required.

 

I am not sure if they will turn the charge off if the voltage gets too high.

Thank you, also helpful 

[jonathanA]

12 minutes ago, IanD said:

How can it disconnect charging and allow discharging when the battery (with internal BMS) only has one set of terminals? Either the cells are connected to the terminals by the MOSFETs (charging and discharging allowed) or they're not...

I belive its called electronics Ian... 🤣😂

 

Sarky comment aside. The frogspawn does do that. I had someone panicking recently... nothing working etc .... turned out his fat finger had caught the discharge off switch on the frogspawn app....

Edited December 4, 2025 by jonathanA

[jessie]

1 minute ago, Jen-in-Wellies said:

Ok, the change should be pretty simple. Replace the gel batts with the LiFePO4 and change the B2B and solar controller settings to their Lithium algorithm. What B2B and MPPT controllers have you got? Make and model. It is usually recommended to limit rhe B2B charge current to around half the maximum alternator current to reduce the risk of the alternator overheating and getting damaged.

Oh, I would never have thought of that, thanks. I’m not 100% sure on model but the charger is renogy and the MPPT is Epever. I think both are 60A. I can check when I find the paperwork, I don’t have it here with me.

 

4 minutes ago, jonathanA said:

I belive its called electronics Ian... 🤣😂

 

Sarky comment aside. The frogspawn does do that. I had someone panicking recently... nothing working etc .... turned out his fat finger had caught the discharge off switch on the frogspawn app....

Are you an electrician @jonathanA?

[IanD]

8 minutes ago, jonathanA said:

I belive its called electronics Ian... 🤣😂

 

Sarky comment aside. The frogspawn does do that. I had someone panicking recently... nothing working etc .... turned out his fat finger had caught the discharge off switch on the frogspawn app....

 

Sorry but it can't -- it can *choose* when to disconnect depending on voltage/current/temperature/charge/discharge/cell balance/fat finger, but once it's disconnected for whatever reason -- while charging or discharging -- it can't do "the other thing" until it has reconnected.

 

So you can't have charging disabled (HV disconnect) and continue to allow discharging into a load, and you can't have discharging disabled (LV load disconnect) and still allow charging. It's both (connected) or neither (disconnected).

Edited December 4, 2025 by IanD

[Jen-in-Wellies]

19 minutes ago, IanD said:

How can it disconnect charging and allow discharging when the battery (with internal BMS) only has one set of terminals? Either the cells are connected to the terminals by the MOSFETs (charging and discharging allowed) or they're not...

The Fogstar supplied BMS can do this. MOSFETS for charging and another set for discharging. Presumably with blocking diodes, like this. Certainly on mine, I can manually turn charging, or discharging on and off, via the app. 

[IanD]

11 minutes ago, Jen-in-Wellies said:

The Fogstar supplied BMS can do this. MOSFETS for charging and another set for discharging. Presumably with blocking diodes, like this. Certainlynon mine, I can manually turn charging, or discharging on and off, via the app. 

I'm afraid your "presumably" is wrong -- you can't have high currents flowing through forward-biased diodes, the power dissipation is far too high (about 100W at 100A).

 

There may indeed be two internal MOSFETS which can block the two opposite cases (Vout>Vbatt=HV disconnect and Vout<Vbatt=LV disconnect), and you can separately tell the BMS when to trigger disconnect depending on charging or discharging (current direction), also temperature (can't charge when cold), and indeed whether to allow charging or discharging at all -- but when the MOSFETs are on they're on (low resistance, a few milliohms), and when they're off they're off.

 

For example if you only permit discharging (charging disabled) then the battery will drive the load, if you then connect a charging source (e.g. MPPT) and ramp the current up the current drawn from the battery will decrease until it hits zero. As soon as the charging current exceeds the load current and current starts to flow into the battery, the battery *will* disconnect. This means it no longer holds the load voltage down, and there's nothing to stop this carrying on rising if the charging source doesn't stop it, because the battery is no longer controlling the voltage at its terminals.

Edited December 4, 2025 by IanD

[jonathanA]

15 minutes ago, jessie said:

 

 

Are you an electrician @jonathanA?

i've been known to dabble  🙂  (yes - although semi retired these days)