I know one of the criticisms here is that people come for advice and then run away without feedback. Well, thanks to some good advice here (and a bit of time with ChatGPT) I have replaced my 9 year old agm batteries with a LiFePo4 ,100 Ah group 24s. I kept an agm battery as a start battery. It was a simpler process than I thought with a bit of guidance and we have enough onboard power now that I am looking for ways to use more. Our Beneteau 323 has quite modest power demands. I was amazed at how light the LFPs are and how back breaking the agms were. I removed the shore charger connection to the house bank (I actually just took the fuse out) rather than swapping for a lithium capable charger, so shore charging is only to the agm starting battery. The LFP house bank charges from solar and via a dc to dc charger from the agm. I know there is some discussion that this is a wrong headed approach and all charging should go the lithium bank but this works well for our use case and was the easiest and least expensive option. We have spent multiple days at anchor with no more than a short motor runs to raise and set the anchor and still had plenty of power. I have more than doubled my available battery power. Because I really have no familiarity with the LFPs as house batteries, so I installed a Victron battery smart shunt to help me figure out state of charge. I'm planning on using the smart shunt to control a solid state relay when I take advantage of my new found power and some excess solar and swap out my ac water heater element for a small dc one. I added in 2 x125 amp MRBF at the positive terminals but I left the original Beneteau switching as it is with the jumper between switch one and two. I have once inadvertently started the diesel with the LFPs without issue. I was in the habit of just using the house bank all the time for everything, so that has been an adjustment. I have 2 discreet charging circuits for the 2 chemistries but the option in an emergency to combine the 2. As we are unfortunately forced to start thinking about haul out and winter storage, I am trying to decide what to do with the LFP batteries. Rather than hauling my agms up and down the ladder, I used to just left them disconnected on the boat. I could probably do the same thing with the LFPs but some winters, we can get ridiculous temperatures for a few days that may damage my shiny new batteries and they are light enough to haul around. All in all, the batteries were cheaper than replacement agms, I have twice the power and they weigh half as much. Pretty happy at this point
LFP for AGM Battery Swap
- Thread starter Phil Mac
- Start date
Thanks @Phil Mac for your in depth reply.
Thanks for setting a good example.
That practice has left me shaking my head for years on this site. Makes me wonder whether the OP gave up the original project or more likely their mothers didn't teach them any better manners.
Thanks for setting a good example.
Based on what Maine Sail has shared with us over the years, doing this means your banks are always combined. Here's his article on why(unless I'm misunderstanding what you wrote)
Reply #8
One black and two red electrical switches
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No they are definitely separate unless you turn both switches on and combine them deliberately. I used to only use my house bank. Bank 2 on my boat is the one connected to the starter. My 285 was wired so that it didn't matter which switch you selected, both banks were in parallel all the time and I was surprised when I bought the 323 that only the bank selected charged from the alternator. The Xantrex charger was connected from the charger to both banks at the switches so when you turned on the charger, it charged both banks. I took the fuse out so that now only the start bank is being charged. The alternator only charges the agm bank. That's obvious from the dc to dc charger.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 23,465
You did not identify the size of your new House Bank. I am guessing there is more than a single new LiFePO4 battery in your house bank. My suspicion is that they are connected in Parallel, so you are getting 12 volts but several hundred Amps - Amp/hours
@Stu Jackson referenced MainSail. Check out the following two links.
Easy LiFePo4 Conversion - Marine How To
Easy LiFePo4 Conversion Before reading this article please read the 101 article on Drop-In's: Drop-In Batteries Be An Educated Consumer Hardly a day goes by where we don't see or hear
marinehowto.com
Drop-In LiFePo4- Be an Educated Consumer
Lead is Dead! (Except for Starting Batteries) Warning: This article is long and detailed—it may require two reads to fully grasp the nuances and important details. You might be wondering why I’m declaring that lead is dead. The answer is simple:
marinehowto.com
I suspect that your MRBF fuses may be woefully under rated.
Each battery weighs ~20lbs? If they are easy to get to, then take them home with you for the winter.
Mark
Mark
They are 2x100 Ah Batteries connected in parallel so 200 Ah. I used a Blue Sea 2 stud MRBF terminal on the first battery. From what I saw, for a bank that size, I could use an MRBF fuse and it seemed like an easy solution. The fuses are rated 25% more than the maximum current. Should I have gone larger?
I added the smart shunt so I could read the state of charge. They do not have a bluetooth BMS. The batteries were a gift but I would have chosen batteries with a bluetooth bus for a lot of reasons.
They are in parallel. 2x 100Ah, 100A smart BMSYou did not identify the size of your new House Bank. I am guessing there is more than a single new LiFePO4 battery in your house bank. My suspicion is that they are connected in Parallel, so you are getting 12 volts but several hundred Amps - Amp/hours
@Stu Jackson referenced MainSail. Check out the following two links.
Easy LiFePo4 Conversion - Marine How To
Easy LiFePo4 Conversion Before reading this article please read the 101 article on Drop-In's: Drop-In Batteries Be An Educated Consumer Hardly a day goes by where we don't see or hearmarinehowto.comDrop-In LiFePo4- Be an Educated Consumer
Lead is Dead! (Except for Starting Batteries) Warning: This article is long and detailed—it may require two reads to fully grasp the nuances and important details. You might be wondering why I’m declaring that lead is dead. The answer is simple:marinehowto.com
I suspect that your MRBF fuses may be woefully under rated.
jssailem
SBO Weather and Forecasting Forum Jim & John
- Oct 22, 2014
- 23,465
Phil. What is the short circuit rating of each of the batteries? In Parallel configuration that rating is cumulative. Your fuse will need to be able to function without welding shut.
@Maine Sail shares this detail on his site:
If you find this helpful in protecting your boat, why not buy the guy a cup of coffee.
BUY ME A COFFEE if you want to support this site.
ABYC standard
The green arrow is pointing to a specification that can only be met by a Class T fuse!
AIC & Why It’s Critically Important!
AIC stand for Amperage Interrupt Capacity. The AIC is sometimes called “interrupt capacity”too. AIC is the short circuit amperage capacity the fuse or breaker can “interrupt safely” without welding shut, blowing up and losing “ignition protection” or jumping the gap and failing to provide protection..
@Maine Sail shares this detail on his site:
Battery Banks & Over Current Protection - Marine How To
Battery Banks & Over-Current Protection A 300A (MRBF)Battery Terminal Mounted Fuse by Blue Sea Systems In this article we will discuss fuse selection, fuse types, wire ampacity, Amperage Interrupt Capacity (AIC) and even potentially unsafe ABYC “exceptions to the rules”. In short,
marinehowto.com
BUY ME A COFFEE if you want to support this site.
ABYC standard
The green arrow is pointing to a specification that can only be met by a Class T fuse!
AIC & Why It’s Critically Important!
AIC stand for Amperage Interrupt Capacity. The AIC is sometimes called “interrupt capacity”too. AIC is the short circuit amperage capacity the fuse or breaker can “interrupt safely” without welding shut, blowing up and losing “ignition protection” or jumping the gap and failing to provide protection..
Doesn't the BMS prevent overcurrent? It is rated for 100 Amps. These are only 100Ah batteries but I appreciate the low internal resistance presents a risk with LiFEPO4 but I was afraid of putting too big a fuse on the circuit as well. I want to make sure it is protected properly. I have the product manual on my boat I'll have a look when we are down this afternoon for short circuit rating. Thanks for the info
I got ChatGPT to run the numbers and this is what I got. Let me know what you think:Phil. What is the short circuit rating of each of the batteries? In Parallel configuration that rating is cumulative. Your fuse will need to be able to function without welding shut.
@Maine Sail shares this detail on his site:
If you find this helpful in protecting your boat, why not buy the guy a cup of coffee.Battery Banks & Over Current Protection - Marine How To
Battery Banks & Over-Current Protection A 300A (MRBF)Battery Terminal Mounted Fuse by Blue Sea Systems In this article we will discuss fuse selection, fuse types, wire ampacity, Amperage Interrupt Capacity (AIC) and even potentially unsafe ABYC “exceptions to the rules”. In short,marinehowto.com
BUY ME A COFFEE if you want to support this site.
ABYC standard
The green arrow is pointing to a specification that can only be met by a Class T fuse!
View attachment 234244
AIC & Why It’s Critically Important!
AIC stand for Amperage Interrupt Capacity. The AIC is sometimes called “interrupt capacity”too. AIC is the short circuit amperage capacity the fuse or breaker can “interrupt safely” without welding shut, blowing up and losing “ignition protection” or jumping the gap and failing to provide protection..
From our earlier PSCC estimate for your two 100 Ah LiFePO₄ cells in parallel (ESR assumptions 3–10 mΩ per cell) we got a prospective short-circuit current on the order of ~2.6 kA → 8.7 kA. The Blue Sea 10,000 A @ 14 V DC rating therefore exceeds that upper estimate, giving a margin of safety
Let's to this yet again:
1. You size the wire based on the current and wire length (voltage drop)
2. You then size the fuse based on the wire size
That's all.
It's all there on the BlueSeas website.
You misunderstand the purpose of the BMS. It is not for overcurrent protection. That is what fuses are for. If you pull more current than your BMS can handle, you'll just "break" your new batteries. This is all discussed in Maine Sail's excellent tutorials.
That’s almost all. When dealing with Lithium Ampere Interrupt Capacity (AIC) matters too. AIC is the maximum amount of current that a fuse can cutoff before the fuse itself welds shut and keeps letting current through. Lithium batteries can put out more amps than traditional lead, so it needs higher AIC rated fuses.
Generally MRBF AIC’s are borderline for lithium and Class T is the preferred solution. I’ve read that there’s an updated ABYC standard for exactly when a Class T is needed but I haven’t seen the new standard for myself so I don’t want to misquote it. I will say this seems like the kind of nuance I wouldn’t blindly trust ChatGPT with, especially since the standards are likely newer than its training set.
I appreciate the fuse is required for overcurrent protection but I wanted to not oversize the fuses to the point the cables or connections could be exposed to a fault current the fuse wouldn't clear and I was not relying on the BMS alone as the only overcurrent protection for wiring — fuses are there to protect conductors and equipment.
I tried to size the fuse to correspond to my cable runs and the battery. According to Blue Seas claim they meet the ABYC requirement. I only used ChatGPT to run the numbers but when I asked it to verify my setup against the revised standard, the fault that came back was that my specific battery model was CE but not UL listed. I don't trust ChatGPT either but it can be useful if you query it for information that it can acquire.
Maybe I should revise that to say, ChatGPT is useful if you query it for information that it can acquires from a place you can trust. I used it when we installed our EV charger to verify our local codes to make sure we were compliant before the inspector showed up.
I agree - I didn’t mean to sidetrack us and broad brush all AI use cases as untrustworthy. I’ve seen it make unfortunate mistakes like conflating volts with amps for fuse ratings before, but with care it can be useful.
Upon more reading of the new ABYC standard based on ABYC publishes updated battery and electrical standards your AIC sounds reasonable to me. Panbo quotes the standard as requiring 5KA AIC for each 100AH of lithium. If your batteries are connected to a positive bus then you have two 100AH batteries and they’re each protected by a 10KA AIC fuse at the terminals, so you should be fine. If your batteries are daisy chained then the “second” battery terminal has a 10 KA AIC fuse and 200 AH of lithium behind it, which is still fine.
You may want to check the size of the cables between wherever they are paralleled and the next downstream overcurrent protection. Since the batteries are in parallel and each one is protected by its own 125A fuse you could put 250A on the cables that carry the total current from the bank. If the cables are big enough to handle 250A then you’re fine. If you thought they only needed to be big enough to carry 125A you may want to reconsider.