I have a setup with both an alternator and a MultiPlus II charging a large bank of dumb (non-communicating) LFP batteries that have FCP (Full Charge Protection). There are no lead batteries in the system. I have both the MP and the alternator regulator settings set so that the LFP bank should never go into FCP, but I’m still concerned with protecting the remainder of the system from over-voltage transients in the event the FCP kicks in anyway. (BTW, simply replacing the dumb LFPs with something smarter is a very last resort).
It has been suggested to me that I place an LTO battery in parallel with the LFP bank, as it acts more like a lead battery in absorbing brief overvoltage spikes. An obvious idea is to add an AGM battery into the system, but I’m unsure whether if I put it behind (eg) an ArgoFET whether it will have the desired load-sink effect. Having the alternator charge an AGM and then DC-to-DC to the LFP gets suggested, but that both still exposes the overall house bank to overvoltage, but also sorely under-utilizes the high output alternator.
If I add in an LTO battery, what size should it be relative to the house bank?
All this depends on the voltage of your system, and the batteries concerned.
a good start would be to use either the cranking battery or a dedicated AGM battery to protect the alternator. For a 12v system, the voltage range for AGM and LiFePO4 batteries are sufficiently similar that a simple ACR (automatic charge relay) can be used to charge the house bank when the engine is running. Similarly, an ArgoFET splitter could be used. For a dedicated alternator battery, the size will depend somewhat on the Alternator current rating, as it must be capable of absorbing the peak output current - on a short term basis. This would typically be somewhere in the 45 - 70Ah range, look at the battery’s CCA rating.
You could look at the Balmar Alternator Protection Device which is meant to avoid the voltage spikes generated by alternators on load disconnect. They may not work 100%, there is one report on here of an alternator being damaged on load disconnect with an APD fitted.
The system is 12V and currently has LFP for both house and starter (the starter LFP has a starter-capable BMS).
If I put a 160amp alternator that I’ll de-rate to about 100a behind a 200a ArgoFET, and then all the batteries behind the ArgoFET, will that dampen any spikes from BMS disconnects?
It would be possible to use a high power zenner diode or other voltage clamp in parallel with a large capacitor. Any surge device would have to be capable of absorbing 100A for a few seconds without the voltage going above 15V, while the regulator and magnetic field in the alternator compensate. This is likely to cost more and not be as effective as a small lead acid battery. The alternative is to replace the polyphase (often 6 phase) rectifier in the alternator with a higher voltage type, and use an external regulator. Not having any protection is going to cause problems.
looking at the Balmar APD, I don’t believe the claim:
. The APM can absorb multiple surges of such energy, and can absorb surges in excess of 200 amps without failure to continually protect your alternator.
As the device does not look big enough or have thick enough leads to achieve this.
There’s already an external “smart” alternator regulator, but it cannot take the field to zero fast enough to prevent a voltage spike in the case of a BMS disconnect.
If I put an AGM into the system behind the ArgoFET, will it have the intended effect, or should it be direct-wired in parallel to the LFP bank?
Yes, the Argo fet can be between the alternator and the battery.
It’s not just the Regulator that has to shutdown the field coil in the alternator, it also takes time for the magnetic field in the Alternator ‘core’ to dissipate.