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Marine battery compartment: don’t place lead acid and lifepo4 in the same box…

curiouscarbon said:
systems that utilize devices lacking low voltage lockout, may benefit from a main under voltage lock out mechanism, which some use a MOSFET BMS to implement. solenoid relays are also common
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On a boat, the best approach is to have a charge and discharge bus with separate BMS driven control. Depending on the battery schema (single dual, triple independent banks) a separate mission critical bus can be implemented. This can drive things like radio, nav gear, bilge pumps etc.

Ideally all charge and loads can be instructed to drop out via a digital com bus. But with so many DC native devices on a boat, this is not likely or reasonable. So the best we can hope for is the inverters and charge controllers to be BMS aware, so they can be throttled or disabled to prevent main bus disconnect.
 
Luthj said:
The major difference is that a lead acid battery can self balance its cells during absorb through the shuttle reactions. Lithium batteries cannot do this. So balancing is needed, and a BMS can perform this.

Lithium cells do not self limit their voltage during absorb. So if a single cell exceeds the max safe voltage, the electrolyte decomposes, causing swelling, possible venting, and internal shorts/fire. Lead acid does not have this issue.

The next difference is that lead acid batteries, while they can be degraded by discharging to empty, will recover, and continue to safely operate. Lithium batteries can be destroyed if allowed to completely discharge, and can result in cell shorts/fire/failure during subsequent recharge.

With a lead acid battery, an external voltage sensing relay can be used to prevent over discharge, and shed any non critical loads. These are pretty robust units, and failures are rare.


With lithium cell level monitoring is required. This requires quite of a bit of wiring, cell connections, and a logic board.
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you're totally correct that lead acid is more resilient to unfortunate situations.

lithium cells are more prone to capacity degradation or spontaneous failure.

it's pretty neat to me that lead acid does not strictly require an auxiliary cell balancing function to maintain inter-cell soc/voltage balance
 
curiouscarbon said:
it's pretty neat to me that lead acid does not strictly require an auxiliary cell balancing function to maintain inter-cell soc/voltage balance
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Its a very nifty feature of the chemistry. It also makes them less efficient when charging unfortunately.

Back when lead acid was used in industrial vehicles, we would occasionally use balancers to keep individual cells (200lb+ 2V cells) in balance. This was typically due to difference in cell ages, and insufficient absorb time to get the lowest cells back up. As these large cells aged, they could have a difference of 2x in self discharge between them, and significant IR differences.
 
lithium cells can hit their respective safety or capacity-degrading voltage limits "more easily" than lead acid. so people aggressively implement low/high voltage disconnect

with a lead acid system, it still makes sense to have a low voltage lockout to me. active/passive cell balancing seems way less important in the context of lead acid cells
 
Luthj said:
Its a very nifty feature of the chemistry. It also makes them less efficient when charging unfortunately.

Back when lead acid was used in industrial vehicles, we would occasionally use balancers to keep individual cells (200lb+ 2V cells) in balance. This was typically due to difference in cell ages, and insufficient absorb time to get the lowest cells back up. As these large cells aged, they could have a difference of 2x in self discharge between them, and significant IR differences.
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interesting! thank you for sharing; fascinating that lead acid banks of differing internal resistance would benefit from auxiliary balancing devices, like from different wear level on each bank

with lithium, the inefficiency seems shifted from chemistry/coulombic efficiency, towards analog or digital circuits that manage the voltage balance
 
curiouscarbon said:
with a lead acid system, it still makes sense to have a low voltage lockout to me.
Click to expand...

Yes, the marine systems I have worked on were all equipped with at least one load shed programmed to leave some reserve capacity.

Folks like to throw around the 50% DOD limit for lead, but thats not very accurate. If you look at the total amp hours delivered from a lead battery vs DOD, discharging to around 80% DOD doesn't yield significantly fewer total AHs delivered.

It does yield fewer overall cycles, but the increased cycle depth balanced most of that out.
 
curiouscarbon said:
interesting! thank you for sharing; fascinating that lead acid banks of differing internal resistance would benefit from auxiliary balancing devices, like from different wear level on each bank
Click to expand...


Indeed. In the early days of EVs, most of us were running lead acid batteries for cost reasons. I was too cheap to buy a new pack every time a battery died, so having a 6A active balancer running on the pack (each battery was 6v, so it wasn't cell level) allowed me to keep the charge rates up to finish absorb without resorting to individual 6V chargers to get the balance back. It wasn't ideal, but saved a lot of money.
 
Luthj said:
a 6A active balancer running on the pack (each battery was 6v, so it wasn't cell level) allowed me to keep the charge rates up to finish absorb without resorting to individual 6V chargers to get the balance back. It wasn't ideal, but saved a lot of money.
Click to expand...
nice! ?
epic solution. rather than move battery, move energy ?️
 
Luthj said:
The major difference is that a lead acid battery can self balance its cells during absorb through the shuttle reactions. Lithium batteries cannot do this. So balancing is needed, and a BMS can perform this.

Lithium cells do not self limit their voltage during absorb. So if a single cell exceeds the max safe voltage, the electrolyte decomposes, causing swelling, possible venting, and internal shorts/fire. Lead acid does not have this issue.

The next difference is that lead acid batteries, while they can be degraded by discharging to empty, will recover, and continue to safely operate. Lithium batteries can be destroyed if allowed to completely discharge, and can result in cell shorts/fire/failure during subsequent recharge.

With a lead acid battery, an external voltage sensing relay can be used to prevent over discharge, and shed any non critical loads. These are pretty robust units, and failures are rare.


With lithium cell level monitoring is required. This requires quite of a bit of wiring, cell connections, and a logic board.
Click to expand...

Not at at all LTO doesn’t need a BMS for example

LFP doesn’t need a BMS either they have largely arrived because such batteries are expensive and are sometimes charged in correctly

LFP consistently charged /discharged in fractional C situations don’t even need regular balancing as they tend to converge not diverge. One a year manual balancing is sufficient.

LFP charging is much much simpler to charge then lead acid. Just charge to a voltage setpoint , set the setpoint conservatively

again both lead acid and li benefit from cell monitoring and many lead acids have Soc monitoring , and load shedding.

Bms disconnect can be required by code but technically using conservative charge and discharge disconnect is not needed and could in fact be manually activated.

The whole alternator disconnect issue is massively overplayed by people who have never experienced one.

Again parallel different chemistries is a bad idea especially Li and AGM and is not supported by several battery manufacturers. Don’t do it. It’s not needed anyway.
 
I feel sorry for the OP in threads like this. His SLA battery got too hot, bubbling acid onto his LFP battery, but it really wasn't the fault of the SLA, or the fact that they were placed in such close proximity to one another. The issue, as I see it, is that he put raw cells (with very good terminal covers) into a compromised environment. If he had put those cells into a proper case, just like the cells in the SLA case next to it, this entire mess could have been avoided.
 
Skypower said:
The only lead acid batteries I’ll have on a boat are AGM’s as starting batteries. They should never be venting caustic fumes into the engine room, bilge or living spaces. I’ve found a radical reduction of corrosion of hardware in and around the engines after going to AGM’s. I’m afraid that the aluminum case on the Lifepo4 cells are compromised. I can’t even recommend an alkali to neutralize the acid spillage because it may do more harm than good. You probably cooked the FLA battery because of overcharging. You need to find the source of the overcharging before you replace them(hopefully with AGM’s. Vmax brand is the best I know of.
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What do you think about using the high current Headway cells for a LiFePO4 starter battery? https://batteryhookup.com/products/12v-handle-beast-module-with-24-headway-38120hp.
(I think you still need a small AGM to protect against an alternator surge if the BMS shuts off, so still not acid-free.)
 
Flint_RV said:
What do you think about using the high current Headway cells for a LiFePO4 starter battery? https://batteryhookup.com/products/12v-handle-beast-module-with-24-headway-38120hp.
(I think you still need a small AGM to protect against an alternator surge if the BMS shuts off, so still not acid-free.)
Click to expand...
There are companies out there building LFP batteries with a large capacitor that will generate the amount of cold cranking amps we expect out of a good AGM. They also have circuitry that allow them to be safely charged from the high-output alternators on large boat engines. Those are the only options I'm aware of, if you want to get away from lead batteries entirely. Personally, I'm fine using an AGM for my cranking battery, as well as running my livewell and bilge pumps. For everything else, LFP is working very well.
 
Something not mentioned and often overlooked. When a flooded battery boils and vents, the GAS is corrosive. It doesn't have to spill liquid acid, just being in proximity will subject nearby metal to corrosion. Before I switched to LFP, I really abused my FLA batteries. I tested and watered them often, and they stayed in decent shape, but bass bars, terminals, my shunt, all nearby in the battery compartment all suffered corrosion.

Just another little reason I am happy to be done with lead.
 
BroomJM said:
While that is a link to Li-Ti battery cells, none of the cells shown is really suitable for building an AGM replacement, at least not in terms of adequate CCA for challenging applications.
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That was also my opinion. The member that posted about them did not provide a link, I was not familiar with them, so I did a quick search and that is what I found.
 
BroomJM said:
While that is a link to Li-Ti battery cells, none of the cells shown is really suitable for building an AGM replacement, at least not in terms of adequate CCA for challenging applications.
Click to expand...
Certainly LTO is way safer and more capable then AGM , a mere review of the specs shows that.

Not sure what a Li-Ti cell is.
 
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