Which reserve battery do you use with LiFePo4?

[john61ct]

Magnum with BMK remote.

Victron with BM + one of their central controllers, but I believe disabled in a firmware update.

Some people use a simple relay, cut off charger input via rPi or Arduino based on reading off a shunt.

[newhaul]

Originally Posted by ronstory

OK, I tried to search the LFP mother thread and could not seem to extract any ah-ha information in the 6K+ responses. But that could just be me (sigh)

Today, our usage is not the typical cruiser. We live 2.5 hours away and have our boat in a great marina local to all the of restaurants so it sits on shore power most of time. For us, it’s mostly about the activity of sailing and not the destination. We typically take day trips and so far have done a just a 3 day trip with a bit of anchoring. Then back to a marina… so in that environment I don’t worry about battery PSOC issues.

That said, I’m a geek. This boat is a great sailing boat, but just the ‘test boat’ for a future larger boat in an area with more consistent wind. The summer in Puget Sound is usually a bob-n-bake with marginal sailing, still enjoyable, but not exciting.

Anyways, I’m building a 200AH LiFePo4 bank from raw cells and I’ll stitch a BMS into it for a full system with LVC and HVC, etc, etc. Yes, my usage doesn’t need it, but it will be fun.

With the charge voltages differences between LiFePo4 and a reserve battery of FLA, Gel or AGM, I could design a charge system with a voltage differential to handle the different types.

However, before I design a wheel… what do the folks with boats using LFP banks use for a reserve battery? More LFP? AGM? Nothing?

ok I read the rest but one question seems to not have been asked .
What is your thinking as to a reserve battery?
If to run the vessels power system on a house bank? why?
If to serve as an oh Crap start battery for the mains then I suggest investing in a small portable jump pack.
Hand cranking gets old fast.

[Rusty123]

I’m primarily a marina sailor too, and have also been thinking about installing a LiFePO4 house bank, mainly as a geek project. I’d also like to keep a reserve starting battery, so I’ve thought about just installing one (FLA or AGM) completely separate from the house bank, and maintaining it with its own small shore charger. In the unlikely case that the house bank failed, I’d use the reserve battery to start the engine and head for the closest marina.

[newhaul]

Originally Posted by Rusty123

I’m primarily a marina sailor too, and have also been thinking about installing a LiFePO4 house bank, mainly as a geek project. I’d also like to keep a reserve starting battery, so I’ve thought about just installing one (FLA or AGM) completely separate from the house bank, and maintaining it with its own small shore charger. In the unlikely case that the house bank failed, I’d use the reserve battery to start the engine and head for the closest marina.

the likely hood of the reverse happening is much more likely.
That your start battery would fail long before your Lfp house bank even begins to loose even 1% of its capacity. Now using a cheap Fla battery as a sacrificial lamb for the possibility of your bms doing an HVC disconnect when charging to protect your alternator s a much better scenario to plan for .

[john61ct]

Proper battery care includes replacing them long before failure becomes likely.

Yes some lead batt online to alt output is required, LFP should only be feeding off that if a proper (expensive) regulator is in between anyway.

The distinction between a robust Starter bank, and a Reserve bank instead, is a pretty grey area.

These finer points are really only of hobbyist interest for a boat that's getting back to shore power on a regular basis.

[Q Xopa]

Originally Posted by john61ct

Magnum with BMK remote.

Victron with BM + one of their central controllers, but I believe disabled in a firmware update.

Some people use a simple relay, cut off charger input via rPi or Arduino based on reading off a shunt.

Ah ok, dont know about Magnum having a Trailing amps end charge.

I have Victron MPPT, BMV and a Sterling Pro and a B2B. I havent found how to do that with any of them yet.

[Q Xopa]

Originally Posted by john61ct

Magnum with BMK remote.

Victron with BM + one of their central controllers, but I believe disabled in a firmware update.

Some people use a simple relay, cut off charger input via rPi or Arduino based on reading off a shunt.

RPi and Arduino are not really standard commercial chargers.

[newhaul]

Originally Posted by john61ct

Proper battery care includes replacing them long before failure becomes likely.

Yes some lead batt online to alt output is required, LFP should only be feeding off that if a proper (expensive) regulator is in between anyway.

.

why do you need an expensive regulator between the Fla start battery charging and the Lfp charging ?

Also the Fla battery in the system is not actually needed but a small 12 v lead battery to protect your alternator regulator is a good idea.

[guitarrich]

I think a second “service” battery is a sensible idea. As one poster mentioned, have a (eg AGM) battery supplying navigation electronics and emergency systems. This can (should) be mounted well above the waterline. It’s charged via dc/dc converter from your main LifePo bank. This gives the dual advantage of short term redundancy plus operating electronics in a severe flood.

I managed a new 24v boat with a house alternator, (the engine was 12v) charging the house LIfePo bank. Initially the setup included a Balmar regulator which was supposed to limit charge current. For reasons nobody understood, it didn’t! Now a Lithium charge curve is “straight”. The battery will suck as hard as it can if allowed to, and do so until it’s full. Three alternators were burnt out before a whole new approach was adopted, (roughly the opposite of my initial recommendation). This works fine.

It was an unsolved puzzle, the failure of the Balmar to regulate charge - it wouldn’t even do it on the temp sensor- attached to the alternator, (which got red-hot). However, nobody amongst the various OEMs and electricians wanted to go down the route of trying a different regulator, following the spectacular failure of the first! It was a miserable few months!

[GrowleyMonster]

Originally Posted by jdaltonpe

LiFePo4 use battery management systems which will shut the battery system down if certain parameters relating to temperature, charge rate, voltage level, etc. are met. Not likely to happen on a well designed system yet still possible. As a full time cruiser I wanted a completely separate backup that could be used for navigation, autopilot, nav lights, etc. on my 12v system. With this in mind I installed a 50Ahr trolling battery with it's own solar panels and charge controller. This battery doubles as a power source for the electric hooka. I am planning on attaching a small inverter to this circuit and using it as a charging station for power tools, electric bicycles, spot lights, etc. In theory, if the main LiFePo bank gets shut down I can trim electrical demand to necessary only items and switch to this independent power source to get through until the main bank comes back on line.

Well, the ultimate backup battery is probably Nickel/Iron Edison cells. You will never as long as you live, have to replace one. They don't "go bad" from over or under charging or long storage or neglect. Every 20 years or so you might have to change the electrolyte (plain old lye, maybe with a bit of lithium compound and "battery oil" which is just colored mineral oil, added) and that's about it. Basically zero "plate" loss. They have poor energy density and a high self discharge rate. Hard to source, and not cheap. But you can forget you even own them and still have them.


My own choice for immediate availability for use would be a pair of golf cart batteries. Cheap and rugged. If you keep them on float and make it a point to actually discharge them to about 70% SOC once a month or so, and equalize 2 or 3 times a year, as a backup they should easily last 10 years. Talking Duracells or Energizers at $85/ea from Sams Club. It's what I use on my electric drive boat, for propulsion.


If you can afford it, why not just install a complete twin setup to what you already got? You can even alternate use days. Same charger, same maintenance, same everything. The only thing better than having a complete backup system is having a complete backup system that is just as capable as the primary system.