AGM>LiFePo4 Upgrade design help

[Statistical]

Originally Posted by sailingharry

That surface mount MRBF fuse block looks like a nifty piece of gear. But I'm not entirely sure how much it gains you.

* In the it unlikely event that you have some sort of a short in the 18-in cable between the battery and the fuse block, it provides no protection at all. But this is an extremely unlikely scenario.

* In the more likely scenario of an overload condition downstream of the block, the first and only fuse to blow will be the 300A fuse.

* In the unique failure mode where you have a load in excess of 200A, and less than 300A, and one battery disconnected for some reason, a battery fuse would blow. Using 2/0 cable between the battery and the fuse would eliminate this concern.

* As drawn, none of these fuses would theoretically ever blow because your BMS would trip first. An argument could be made that the fuses should be low enough to protect the BMS.

I would personally look at the system as drawn as adding complexity, cost, and space with very little return on that investment. I would size the battery cables to individually support the 300a fuse (2/0), and only have the one fuse.

EDIT: I just realized there is one failure mode that might benefit from this system. Some sort of internal short in one battery (that somehow doesn't cause that one battery to detonate!) would cause the second battery to unload through the failed battery. This would be very high current with no protection. The battery fuses would protect this failure mode. If it is even a legitimate failure mode.

It is the second 300A fuse which is redundant just simply size wiring to handle the full 400A at that point. Fusing at each battery is ideal if possible especially with LFP.

You quickly can run into a situation where a single fuse does not have the required AIC to handle a dead short for multiple batteries. MBRF fuses have a 10,000 A AIC. A 100 Ah LFP cell tend to have internal resistance of 0.4 mOhm which is a short circuit current of 9,125A (at peak 3.65V). MBRF covers that. Multiple cells in series (i.e 4 for "12V" battery) don't change the short circuit current. However put two in parallel and now we are taking 18k A.

The low capacity and high internal resistance of lead acid batteries means you can get away with a lot of poor practices. LFP are serious and should be treated as such. Don't rely on the BMS saving you in a high current fault Mossfets can fail closed allowing unlimited current flow. Even contactor based systems the contactor can weld closed under extreme current.

The risk of a short in the short length parallel cables is low but not zero. There is no need to take that risk. Protect each battery separately with its own fuse. For smaller drop in replacements MBRF is fine. For larger ones, DIY large cell or parallel cell configurations or just more peace of mind go with Class T.

[donradcliffe]

1. running the ground to the lead keel is asking for galvanic corrosion.

2. I'd repurpose the ground switch as an emergency battery combiner switch for when your start battery fails.

3. Your start battery may be 65 amphr, but it is rated at 800 amps. This is where the ABYC fuse rules REALLY don't make any sense.

4. By putting a switch on your alternator output, you have created a way for an idiot to blow your alternator if he/she starts your engine without closing the switch to the house battery.

[Scubaseas]

Originally Posted by donradcliffe

1. running the ground to the lead keel is asking for galvanic corrosion.

2. I'd repurpose the ground switch as an emergency battery combiner switch for when your start battery fails.

3. Your start battery may be 65 amphr, but it is rated at 800 amps. This is where the ABYC fuse rules REALLY don't make any sense.

4. By putting a switch on your alternator output, you have created a way for an idiot to blow your alternator if he/she starts your engine without closing the switch to the house battery.

I would be reluctant to just wire in a battery connect switch from LFP house to AGM start battery. Your BMS may not like the idea. A "smart" jumper box would be a safer bet and could be used on either house or start battery chemistry. The AGM won't care what it gets jumped with or connected to but the LFP may disagree with you.


re:4 If you have no power on the alternator B+, nothing will happen to the alternator if the engine is started or running. Removing power from the B+ terminal while the field wire (DF) is energized and the engine is running can fry things by a leading to a big inductive voltage spike. If you use a battery switch that has a wired in field disconnect you can avoid this problem.

[Senojev]

More of a couple of questions for my own design for a house bank that includes two lithium in parallel to positive and negative bus bars using 4/0 cable. The cables will be equal lengths and about 18" long. Can I can fuse the positive with a Class T after the bus bar if I run the cables from the battery in conduit? The alternator will charge the house with a DC to DC charger to charge the start batteries. Do I need alternator protection in the unlikely event of a BMS shutdown? Seems that only one of the BMS would shut down and the alternator would still charge the other battery?

[Statistical]

Originally Posted by Senojev

More of a couple of questions for my own design for a house bank that includes two lithium in parallel to positive and negative bus bars using 4/0 cable. The cables will be equal lengths and about 18" long. Can I can fuse the positive with a Class T after the bus bar if I run the cables from the battery in conduit?

You can but by ABYC it needs to be within 7" of the batteries. However I question if you need that at all. Just fuse each battery and then make sure the cable downsteam of where they are paralleled can handle the combined amperage.

If your post busbar cable can handle 300A then each battery fused to 150A (or less) is fine. If your post busbar cable can handle 200A then each battery fused to 100A (or less) is fine.

The only time you need an additional fuse is if you are narrowing the ampacity. As an example for electric propulsion you have four 500Ah batteries. You limit each of them with a 400A fuse and they are paralleled together at a 1000A busbar. You need an additional fuse at the busbar because the combined allowed ampacity is 1,600A but the barbar can only handle 1000A.

Simple version is put a fuse on each battery for maximum protection. If the batteries are relatively small (<200 Ah) MBRF is likely fine. Beyond that you should use class T or at least compute the short circuit current to verify MBRF is ok.

Your diagram may be unclear in that you are using A instead of Ah for batteries. The intended A output of the battery and it is capacity in Ah may be different. If your diagram is correct and each battery BMS is limited to 100A I see no reason you need anything beyond a 100A fuse on each battery and aproriately sized cables (cables to handle 100A prior to being joined in parallel & 200A afterwards).

The alternator will charge the house with a DC to DC charger to charge the start batteries. Do I need alternator protection in the unlikely event of a BMS shutdown? Seems that only one of the BMS would shut down and the alternator would still charge the other battery?

Need? No. It is just added insurance and pretty cheap. Honestly despite all the fears about BMS disconnect destroying an alternator I have never heard of that happening. BMS disconnects are rare and like you point out BMS disconnect of all batteries when there are 2+ in parallel is even rarer. Still you have a pretty nice alternator and an alternator protector is pretty cheap insurance.

[Capt Ben]

Thank you to all for the great input. Here's a revised layout. NOTE all primary load wire as indicated is 2/0 and lengths are quite short.

1. Reduced MRBF battery fuses closer to max battery spec.*
2. Eliminated keel engine bond
3. Added DC>DC output 60a fuse
4. Labeled many devices
5. Added Alt Regulator sensors (Battery temp & Alt Temp)
6. Added Alternator Protection Module

* Peak discharge current per battery is 200A

[LiFeTech Energy]

Not sure why you don't keep it simple and stick to LiFePO4 for both engine cranking and house power. It is a lot simpler than mixing two completely different chemistries ie, lead acid and lithium with their very different charging voltages and characteristics.

[MVDarlin]

I recently combined my house & start bank into one large 500 ah agm bank, solar powered. It runs my refer and house needs. I added a separate solar powered 200 ah lithium battery to run a chest freezer and StarLink Sat internet. It works well. Note: BMS on LiPO battery limits sustained output to 100 amps so not suitable for over 1,000 watts. Not good for winch or high watt inverter. I have 11 panels. Peak output 750 watts.

[LiFeTech Energy]

In an all lithium installation you need to use a diesel engine starting lithium battery for high power loads such as engine cranking, running high power winches, electric bow thruster motors, high power inverters and the like (this battery is a dedicated battery and used in addition to a separate battery bank for house power).

High discharge rate diesel engine starting batteries are very different to low discharge rate batteries used for house power. Each battery type must be used for their specified applications.

[LiFeTech Energy]

Alternatively there are "2 in 1" lithium batteries which combine both the engine cranking battery and house power battery in the same battery box. This is why these batteries have more than two battery terminals. Attached are a couple of boat owners examples. For example on one of these batteries the house power output is rated at 120A discharge while the engine cranking output is rated at 2800A.

[badadim]

Finished similar (LFP+AGM) upgrade last year. Love it so far.

I see you are using Victorn. Have you looked into Wakespeed WS500 (https://www.wakespeed.com/product/ws...tor-regulator/) and REC-ABMS (https://rec-bms-na.com/)?

Also, you may check Cirix-Li-CT instead of DC2DC charger: https://www.victronenergy.com/batter...tery-combiners

Glad to share more details and answer any questions.

[burrowsmark]

Originally Posted by Capt Ben

I think I'm real close to a proper safe layout design for my upgrade from (2) AGM banks to (1) starter AGM bank & (1) house bank of LiFePo4. I have completely isolated the Start & House banks to make this possible. The Balmar 618 External Alternator regulator as well as the Phoenix Smart charger will all be set for LiFePo4 battery profiles. The DC>DC will be set for AGM.
Any constructive advice, suggestions, or thoughts are most welcome!

Hello,

I see a conflict with the to chargers, you dont want the main charger be in parallel with the dc/dc charger. You could leave the fuse out? So the dc/dc charger takes care of the engine battery.
Also, I would do away with the old amp hr meter shown and consider a new Victron bmv712 meter.

San Diego
Mark