Yet another LiFePo4 query

[LeslieTroyer]

If I switch to Lithium chemistry and charging profiles, and have 2ACRs feeding the start and windlass AGM batteries, is there sufficient buffer to keep my diodes from frying? And 2) how am I hurting the life of the AGMs ? Should I replace the ACR with B2B chargers?

 

[Johann]

It sounds like all charging sources are going to the house and then thru the ACR to the start and windlass batteries? Typical ACRs are not suitable for use with the voltages of LFP. The higher voltage of the LFP will close the ACR and combine the banks until the LFP is about dead. I think there may be LFP compatible ACR, but B2B would be a better choice.

 

[mermike]

It's an interesting question as to whether the load of a B2B charger would keep the diodes from frying if your LFP bank disconnected.

 

[LeslieTroyer]

It sounds like all charging sources are going to the house and then thru the ACR to the start and windlass batteries? Typical ACRs are not suitable for use with the voltages of LFP. The higher voltage of the LFP will close the ACR and combine the banks until the LFP is about dead. I think there may be LFP compatible ACR, but B2B would be a better choice.

Thanks - yes everything goes from/to the house. One could probably hook the Ignition inhibit of the ACR up to the charger or alternator some how, but that would probably still result in fried diodes. Is there a marine BMS that can cut the field before it disconnects ??

 

[Stu Jackson]

Les, there's a ton of info over on CF on this stuff. here's one of many, they have an entire section on liths.

Do I just need a DC-DC charger to charge lithium? - Cruisers & Sailing Forums

I have a small sailboat. Just a single lead acid starter battery and a single lead acid house battery . I was looking to replace the lead acid house battery with a lithium . To charge the lithium , do

www.cruisersforum.com

My take on this is AO >> lith house bank = BAD for the alternator. AO never interrupted >> FLA = OK, need to find the right method to transfer to liths.

 

[Johann]

Is there a marine BMS that can cut the field before it disconnects ??

Yes, I have Lithionics with internal BMS and it has the alternator field disconnect. That being said, I run my dumb alternators to the AGM start batts for now and charge the LFP from there with the B2B. But there are SO many options in terms of cost and complexity it’s hard to give specific advice. I would say the main issues to address are:
1. Protect the LFP from overcharge. The BMS typically doesn’t do this. It protects from over/under voltage, over temp, but not generally overcharge which can occur at 13.7v
2. Protect the alternator from overheating and from BMS disconnect.
3++ Somethings I forgot I’m sure

 

[IStream]

@LeslieTroyer are you planning to roll your own, buy an engineered system with an external BMS, or use drop-ins? Most any system you build or buy with an external BMS will provide SOC and early warnings before the BMS will shut off the contactor(s) to the batteries.

I have a LiFePo house bank without a starter bank and used those signals to drive solid state relays that cut off the ignition voltage to the alts' voltage regulator, shut down the solar, etc. For belt-and-suspenders, I also put some transient clamping diodes on the alt outputs but they should never be needed if the cutoff system is working properly.

 

[LeslieTroyer]

Well I decided to go with 3 drop in lithium and 1 AGM 12v 100AH batteries. I got a Sterling 60A battery to battery charger, and a Sterling alternator protector. I’ll charge the single AGM from the alternator and from there use the b2b to charge the Li batteries and ACR’s to charge the windlass and start. I’ll also add some interconnects/disconnects and alternator disconnect. Thanks all.

 

[Maine Sail]

You CANNOT use an ACR with LFP..

 

[Hayden Watson]

My plan for LFP is to continue to have the alternator output go to my battery combiner with it connected to the agm start and LPF house banks.
I have an old school diode-based battery combiner that is rated for 120A. I have heard many people say they are bad because of the 0.7v drop across the diode buy my battery sense wires are connected directly to house battery so with the regulator set to 14.2v, the alternator puts out14.9v and the battery combiner knocks it down the the required 14.2v.
In switching to LFP, I will change the sense wires from the house to the start so that the alternator will be controlled by the AGM which cannot be disconnected.
Is my understanding of the situation and logic faulty anywhere?

 

[LeslieTroyer]

You CANNOT use an ACR with LFP..

The ACR’s are connected to a 100ah AGM which is charged by the B2B from the LFP’s.

 

[Maine Sail]

The ACR’s are connected to a 100ah AGM which is charged by the B2B from the LFP’s.

in ACR is specifically designed for lead acid voltages, it cannot be put between a lithium iron phosphate battery and a lead acid battery, was my point. if the ACR is between two different AGM's or AGM and another lead acid bank then it's fine.

 

[Hayden Watson]

in ACR is specifically designed for lead acid voltages, it cannot be put between a lithium iron phosphate battery and a lead acid battery, was my point. if the ACR is between two different AGM's or AGM and another lead acid bank then it's fine.

Why is this? I do not know fully how an ACR works and if there are different types. My (admittedly simple) understanding is that they open [joining two banks] whenever the voltage rises above a certain voltage and close [separating the banks] when the voltage drops below a different voltage.

Is this correct and why would that prevent the mixing of AGM and LFP if the charge voltages are compatible with both?

 

[heritage]

Let me see if I have this straight: You have 3 lfp batteries, one agm 100a battery, one start battery, and one windlass battery. And you want to charge the agm battery with the alternator and charge the lfp batteries with the B to B charger from the agm battery. The windlass and start batteries will be charged via an ACR between them and the agm battery. Is that right?

 

[Hayden Watson]

You CANNOT use an ACR with LFP..

I just read Maine Sail's article about ACR's and think I might have figured out why they cannot be used with LFP. They do not disconnect until the voltage on one of the banks drops to 12.75V for 30 Seconds or 12.35V for 10 Seconds. At this point the LFP would be almost completely dead. I have been having a lot of people tell me that I need to ditch my old school diode combiner (that has the sense wires on the battery after the diodes rather than at the alternator) and replace it with an ACR. I agree with Rod that the ACR will not work with LFP due to the ridiculously low disconnect voltage. It looks to me like my old diode combiner will work perfectly with the regulator sense wires on the AGM start battery and the LFP bank on the opposite leg.