LiFePo4 bms?

[Kalimniosjohn]

First off I know LiFePo4 has been talked about a lot, but the mobile search sucks, and living aboard, my phone is my main internet browsing instrument. Also the main threads are soo long it’s difficult to dial in on the information I’m seeking.

So My current agm’s are toast and my gf and I have decided to go with lifepo4 as the next step. I’m going to go with 16 CALB 200 ah cells that are available from aliexpress for around $2500 U.S.. That would leave me with 800 ah 4s4p 12v bank. I’d have one string up in the bow for the windless, and 3 strings midship. Though all 4 strings would be wired together in parallel.

Now that you know what my setup will be I’ll get on with my questions. First will it be okay to eliminate the starting battery and start the perking 4.236 from the house bank, or should I keep a separate starting battery? If it’s best to keep a starting battery, can I just run a DC/DC converter to charge it? What’s the best way?

Second, where is everyone getting their BMS systems these days? Are there any options in the $50-$100 range? I was leaning towards REC but it seems I’d need to buy 4 of them.?. That’s just not cost effective to me as then buying a drop in replacement would be about the same cost. Is BMS even necessary? I could get a victron battery protect for the low voltage cutoff, and program the victron smart solar charger and the AC charger to the correct charge voltage and all I’d have to do is monitor the cells for balance and possibly temperature. Is this correct? I plan to charge to 90% soc and discharge to no more then 20% soc to extend the life of the bank for as many cycles as possible. Realistically I won’t get that low based on my current amp hour usage. Around 250 ah/day. I just want the extra capacity for while offshore and running things like autopilot and radar all night. I believe my autopilot peaks around 10 amps.

What am I missing?

[CatNewBee]

Originally Posted by Kalimniosjohn

First off I know LiFePo4 has been talked about a lot, but the mobile search sucks, and living aboard, my phone is my main internet browsing instrument. Also the main threads are soo long it’s difficult to dial in on the information I’m seeking.

So My current agm’s are toast and my gf and I have decided to go with lifepo4 as the next step. I’m going to go with 16 CALB 200 ah cells that are available from aliexpress for around $2500 U.S.. That would leave me with 800 ah 4s4p 12v bank. I’d have one string up in the bow for the windless, and 3 strings midship. Though all 4 strings would be wired together in parallel.

Now that you know what my setup will be I’ll get on with my questions. First will it be okay to eliminate the starting battery and start the perking 4.236 from the house bank, or should I keep a separate starting battery? If it’s best to keep a starting battery, can I just run a DC/DC converter to charge it? What’s the best way?

Second, where is everyone getting their BMS systems these days? Are there any options in the $50-$100 range? I was leaning towards REC but it seems I’d need to buy 4 of them.?. That’s just not cost effective to me as then buying a drop in replacement would be about the same cost. Is BMS even necessary? I could get a victron battery protect for the low voltage cutoff, and program the victron smart solar charger and the AC charger to the correct charge voltage and all I’d have to do is monitor the cells for balance and possibly temperature. Is this correct? I plan to charge to 90% soc and discharge to no more then 20% soc to extend the life of the bank for as many cycles as possible. Realistically I won’t get that low based on my current amp hour usage. Around 250 ah/day. I just want the extra capacity for while offshore and running things like autopilot and radar all night. I believe my autopilot peaks around 10 amps.

What am I missing?

You need one BMS if you not build 4 separate batteries but one big. 4P4S.

There are cheaper cell modules and other options, ECS, 123BMS etc. But REC was the one for me, it is totally configurable and IP67 protected, important in wet environments.

[Kalimniosjohn]

In order to build one large battery wont I need larger capacity cells? I’ve read that it isn’t good to go with cells over 200ah because the internal mass of the battery isn’t supported well and damage can be caused from vibration and shock loads.

Also, I need at least two batteries so the windlass has a power supply close by.

Edit...........
I just realized what you meant about 4p4s instead of the other way around. Sounds much better.

But what to do about the windlass battery?

[GRIT]

You're not going to need a windlass battery. Size your cables to handle the load, at distance, and you'll be better off.

One large "battery" instead of four small ones. Easier to manage, and much easier to keep track of individual cell voltages.

You use the same cells. 4 in parallel and 4 of those packs in series. Same cells, just one big happy family of cells, instead of 4 families of cells. The 4 in parallel act as "one cell" So you'll only have 4 "cells" to monitor.

We have a similar situation. 16 cells of 180 ah each. To make one 720 ah "battery". We went with the REC BMS.

Cheers.
Paul.

[Kalimniosjohn]

A 100 amp load over 30 feet is going to take some serious wire. I’m worried this could potentially shorten the life of the motor.

As far as the REC system goes, what all did you need to get it up and running? I emailed them Thursday and still no response.

Did you also have to buy the PC software and connections, or can I configure it all from Bluetooth or the display? Did you purchase the precharge unit?

[CatNewBee]

Originally Posted by Kalimniosjohn

A 100 amp load over 30 feet is going to take some serious wire. I’m worried this could potentially shorten the life of the motor.

As far as the REC system goes, what all did you need to get it up and running? I emailed them Thursday and still no response.

Did you also have to buy the PC software and connections, or can I configure it all from Bluetooth or the display? Did you purchase the precharge unit?

Look in my thread, REC BMS, second temp sensor, wire kit all contacts wired, software, optional display. Mine has no Bluetooth (older version) .

It depends on your project and what solenoids you are going to use, I use the BlueSea ML-RBS (see interface description in my thread) . No precharger. You can use the Kilowac solenoids, but they draw constantly power when on, the ML-RBS are bi-stable and draw only current when switching from one side to the other, also can be switched manually, big advantage.

[Steve_C]

Originally Posted by Kalimniosjohn

A 100 amp load over 30 feet is going to take some serious wire. I’m worried this could potentially shorten the life of the motor.

As far as the REC system goes, what all did you need to get it up and running? I emailed them Thursday and still no response.

Did you also have to buy the PC software and connections, or can I configure it all from Bluetooth or the display? Did you purchase the precharge unit?

For 100 Amp load at 12 volts at 30 feet you would only need #2 AWG. That's not really very large. If you went to #1 you even have a safety margin. With appropriate conductors and adequate bank you will not shorten the life of the motor. If your REALLY worried use 1/0 and it would still be far less $ and trouble than having separate banks.

[GRIT]

If you split your system into four batteries, as you had previously suggested; you'd need the same size wire anyway, unless you have a charge source for each battery. Otherwise, you'll never get them close to the same voltage, and it would become a management nightmare.

Everything I know about REC BMS, I learned from CatNewBee, so I have nothing to add there.

Cheers.
Paul.

[Kalimniosjohn]

Thanks for the info guys, all points make sense so far. It will definitely make life easier to go with one battery!

Wow, I’ve been reading your thread CatNewBee...makes me realize how much I have to learn before attempting this. I don’t understand half of what’s going on there.

[Flod]

Lot of good advices. I would suggest to keep the old start battery to dump the current to when charging of the alt. I use this a lot since the lifepo4 tends to get full quite quickly when motoring. This will save your diodes. I guess dc-dc could work depending which way you set it up. Probably someone know more about that particular issue than me. Good luck you will love your new batteries!

[john61ct]

If you do decide to go for separate 12V banks, the windlass at 1P4S, House at 3P4S, only paralleling 2x batteries is much better than four, and you would need 2x BMS not 4x.

But **much** better to just build one big 4P4S bank,

the beefy-enough wiring to support your windlass within tolerable voltage drop over the distance is a one-time infrastructure investment, pays for itself

simpler thus more reliable / likely longer-lived bank, also lots cheaper if you go with a pricey BMS, only need one, no DCDC charger needed.

Look at the Circuit Wizard app from Blue Sea and the voltage input spec of your windlass, to avoid overkill on the wire gauge - but do round up.

Measure the actual wiring path with a rope, double length for a round trip. Assume say 13.1-2V at the input to the wire run, since there will be some sag and the bank may be in the mid-SoC range.

[CatNewBee]

Originally Posted by Kalimniosjohn

Thanks for the info guys, all points make sense so far. It will definitely make life easier to go with one battery!

Wow, I’ve been reading your thread CatNewBee...makes me realize how much I have to learn before attempting this. I don’t understand half of what’s going on there.

Regarding your windlass problem. Mine is about 7m away from the batteries, connected by thick wires by the manufacturer using the GEL house batteries and supported by the alternators running.

No problem with the LFP so. If you really cannot run adequate wires and want to have a local battery at the windlass, you can just place a 50Ah lead acid SMF start battery or AGM battery next to the windlass, and connect it via 2 circuit breaker (fuses) at each battery plus (LFP AND FLA) and and battery switch or solenoid somewhere inbetween to the house bank.

LFP batteries stay at 13.3..13.4V what translates to float in FLA, so the FLA will be always full when connected, she will support locally the windlass to prevent the power drop.

When you leave the vessel long time unattended, disconnect the FLA battery to not drain the LFP house bank, the fuses must be able to tolerate the current along the wire and the windlass, they protect a shot to each source (battery) and also a fatal cell short of the FLA. If your BMS cuts the current, the FLA will provide some energy for the house until it id depleted. You dont need a fancy B2B charger for that cheap battery, simply renew it after 5 years or so, she is most of the time fully charged and not used anyway.