House battery bank only, with jump starter pack backup. Yes/No?

[2airishuman]

At present I run two AGM banks that are ordinarily kept in parallel via a 1-2-both switch. Being able to isolate one bank makes it possible to go on if there is a shorted cell (common failure mode). Same idea probably applies to LFP banks though the failure modes are different.

I am moving towards a supercap for engine starting and suggest you take a look at those. Marine application-specific ones are available but costly, the components are much cheaper at Digikey etc if you know what to look for, and prices for complete systems are likely to come down. Supercaps have many advantages. Among them are support for intermittent high loads without the MOSFET problem, extraordinarily long life (outlast the engine if properly specified and kept reasonably cool), and ability to recharge relatively quickly from trickle sources e.g. solar to recover from an electrical failure. They also inherently limit cranking time so that your soon-to-be-ex won't hydrolock the engine by cranking it for 30 seconds with the stop knob pulled out.

 

[kent_island_sailor]

This thing cranks my engine just fine. FYI

 

[mckenzie.keith]

Oh geez. I hadn't realized the drop-in BMS's worked differently from the stand-alone models. My (apparently faulty) thinking was that having a separate BMS in each battery meant that one wouldn't have to worry about a single BMS failure shutting down your entire house bank (as has been mentioned as a concern elsewhere in this thread). My setup uses Relion drop-ins.

So, a question. How much bank capacity would be needed to provide sufficient capacity "buffer" that MOSFET failure would be unlikely? My smallish (32 HP) 3-cylinder diesel wouldn't seem too demanding to start.

I think the relion datasheet can be trusted. Relion seems to be a decent brand. If they recommend use for starting then all should be well. Which relions do you have? The RB100 makes no mention of starting, but the RB100-HP does mention starting. Presumably the RB100-HP BMS has been designed with a BMS that can handle the starting amps.

EDIT: the RB100 lists the maximum continuous discharge as 100 Amps. Peak discharge of 200 Amps for at least 5 seconds. I would think two in parallel will probably start your motor reliably with comfortable margin, since your starter probably doesn't need more than 200 Amps. If it does, you can use smaller wire to restrict it, LOL.

 

[mckenzie.keith]

This thing cranks my engine just fine. FYI

View attachment 596684

That is so small you could tuck it away almost anywhere.

 

[Panope]

Here is a group 31 size, 280 amp hour, 1000 CCA Dual purpose battery. Not cheap.

Dakota Lithium LiFePO4 280 Ah 12V Dual Purpose Battery Plus

Dakota lithium 12v battery with 6X power of a group 31 marine battery. deep cycle energy plus 1,000 CCA of engine starting power for the 11-year life.

dakotalithium.com

 

[SVArcturus]

I think the relion datasheet can be trusted. Relion seems to be a decent brand. If they recommend use for starting then all should be well. Which relions do you have? The RB100 makes no mention of starting, but the RB100-HP does mention starting. Presumably the RB100-HP BMS has been designed with a BMS that can handle the starting amps.

EDIT: the RB100 lists the maximum continuous discharge as 100 Amps. Peak discharge of 200 Amps for at least 5 seconds. I would think two in parallel will probably start your motor reliably with comfortable margin, since your starter probably doesn't need more than 200 Amps. If it does, you can use smaller wire to restrict it, LOL.

The RB-100s. The LT and HP versions came after I'd bought mine. Those RB100-HP versions are 100A continuous and 800A peak. Sheesh.

Plus, the replacement of the starting battery means I would have three 100A batteries in the house bank. So, 300A continuous, 600A for 5 sec or so.

I was concerned that maybe the MOSFET failure was something that came from the number of heavy amp uses rather than just the size of the draw. If their failure isn't a problem for loads within their stated operations specs (which does sorta makes sense), then I'm comfortable that I'd be fine. The parallel batteries should provide plenty of leeway.

One thing I had thought of until the thread was the fact that using Li batteries for starting means you get pretty much the same voltage and amperage from a battery at 90% capacity as you do from one at, say, 10% capacity. You just can't crank as long. That's a somehow intriguing difference from lead acid.

 

[IStream]

Here is a group 31 size, 280 amp hour, 1000 CCA Dual purpose battery. Not cheap.

Dakota Lithium LiFePO4 280 Ah 12V Dual Purpose Battery Plus

Dakota lithium 12v battery with 6X power of a group 31 marine battery. deep cycle energy plus 1,000 CCA of engine starting power for the 11-year life.

dakotalithium.com

I'm skeptical. Most other 12V group 31 Group 27 LiFePO4 batteries are in the 100-135Ah range. There's a finite internal volume to a group 31 case that, combined with the volumetric energy density of lithium cells, imposes an upper Ah limit that's under half what Dakota is claiming.

Bottom line: this seems way too good to be true.

EDIT: Confused my Group 27 and Group 31s...

 

[mckenzie.keith]

I haven't analyzed it but the wiring may cause the batteries to contribute unequally to starting loads. The wiring resistance may become non-neligible with 100 or more amps. Not saying this will be a problem. I am just thinking out loud.

 

[IStream]

The RB-100s. The LT and HP versions came after I'd bought mine. Those RB100-HP versions are 100A continuous and 800A peak. Sheesh.

Plus, the replacement of the starting battery means I would have three 100A batteries in the house bank. So, 300A continuous, 600A for 5 sec or so.

I was concerned that maybe the MOSFET failure was something that came from the number of heavy amp uses rather than just the size of the draw. If their failure isn't a problem for loads within their stated operations specs (which does sorta makes sense), then I'm comfortable that I'd be fine. The parallel batteries should provide plenty of leeway.

One thing I had thought of until the thread was the fact that using Li batteries for starting means you get pretty much the same voltage and amperage from a battery at 90% capacity as you do from one at, say, 10% capacity. You just can't crank as long. That's a somehow intriguing difference from lead acid.

The 100A continuous rating on each of these batteries is due to the heat buildup on the MOSFETs and/or the cells. With three batteries wired in parallel, you'll have 300A available continuously and starting will be no problem. As you point out, even with your batteries at 20% SOC, they'll still be delivering current at 13V or so, which will make your starter motor happy. FWIW, my 3.0L Yanmar 4JH3-HTE has absolutely no problem cranking over even in the cold with 3 group 31 Group 27 lithiums.

 

[mckenzie.keith]

Let's see. A group 31 is 12.9" x 6.75" x 9.27". That is 327 mm x 171 mm x 245 mm.

280 Ah prismatic LFP cells from Eve are 173 x 72 x 208 mm. If we multiply the 72 mm by 4 that brings it up to 288 mm. Looks doable to me. No problem. They probably just put prismatic cells inside.

 

[SVArcturus]

I haven't analyzed it but the wiring may cause the batteries to contribute unequally to starting loads. The wiring resistance may become non-neligible with 100 or more amps. Not saying this will be a problem. I am just thinking out loud.

I wired the house bank originally to handle 200A w<3% voltage drop. The engine circuit is 2/0 cables for a total circuit run of about 8'. I think it turned out it would handle 280A before exceeding 3% drop.

 

[Panope]

I'm skeptical. Most other 12V group 31 LiFePO4 batteries are in the 100-135Ah range. There's a finite internal volume to a group 31 case that, combined with the volumetric energy density of lithium cells, imposes an upper Ah limit that's under half what Dakota is claiming.

Bottom line: this seems way too good to be true.

I agree. It does seem unbelievable.

However, it weighs 65 pounds while a Renogy group 31 (100ah) only weighs 29.

Also, one of our local Marine tradesman has been installing Dakota batteries in his customers boats with great results.

Time will tell.

 

[mckenzie.keith]

I wired the house bank originally to handle 200A w<3% voltage drop. The engine circuit is 2/0 cables for a total circuit run of about 8'. I think it turned out it would handle 280A before exceeding 3% drop.

I think if you have one battery with 8 feet of 2/0 and another battery with 2 feet of 2/0 cable, there is a possibility that the battery with only 2 feet of cable will contribute more current during starting. I am definitely not trying to say that 2/0 is inadequate for a starter circuit. It definitely is. I am too lazy to do any real analysis or calculations though. I think with three batteries you are going to be fine.

 

[mckenzie.keith]

I agree. It does seem unbelievable.

However, it weighs 65 pounds while a Renogy group 31 (100ah) only weighs 29.

Also, one of our local Marine tradesman has been installing Dakota batteries in his customers boats with great results.

Time will tell.

The 280 Ah Eve cell weighs 5.4 kg. So with 4 of them that would be 21.6 kg = 47.5 lbs. So, again, doable. The rest of the weight is probably the case and BMS and bus bars and whatnot.

 

[Panope]

The 280 Ah Eve cell weighs 5.4 kg. So with 4 of them that would be 21.6 kg = 47.5 lbs. So, again, doable. The rest of the weight is probably the case and BMS and bus bars and whatnot.

That's good news.

As part of my Keel Tank project, I am relocating the house bank with the idea that the final arrangement be a single group 31 LFP battery (I currently use 2 group 31 Lead Acids).

The $2,400 price is to rich for my blood. Maybe in 5 years or so.

 

[mckenzie.keith]

That's good news.

As part of my Keel Tank project, I am relocating the house bank with the idea that the final arrangement be a single group 31 LFP battery (I currently use 2 group 31 Lead Acids).

The $2,400 price is to rich for my blood. Maybe in 5 years or so.

Yeah it is a lot! I can get 4 eve 280 Ah cells for 850 bucks with free delivery from amazon. I also got a quote from thundersky winston for 4 x 300 Ah cells. It is $1430 (US). I am having trouble seeing how Dakota, Battleborn, Mastervolt, Lithionics, all of them get the price so high. If you just string 4 cells together, they are already cheaper than lead acid batteries I think. Does it really cost that much to assemble the cells into a pack and add the BMS and put it in a case? Maybe it does. Maybe they just don't want to have all that stuff done in China where it could be done cheaper. I guess I shouldn't complain if it is saving American jobs?

 

[Fah Kiew Tu]

Yeah it is a lot! I can get 4 eve 280 Ah cells for 850 bucks with free delivery from amazon. I also got a quote from thundersky winston for 4 x 300 Ah cells. It is $1430 (US). I am having trouble seeing how Dakota, Battleborn, Mastervolt, Lithionics, all of them get the price so high. If you just string 4 cells together, they are already cheaper than lead acid batteries I think. Does it really cost that much to assemble the cells into a pack and add the BMS and put it in a case? Maybe it does. Maybe they just don't want to have all that stuff done in China where it could be done cheaper. I guess I shouldn't complain if it is saving American jobs?

I think more likely the price is that high because that's what the current market will bear. On another forum I hang out on, the phrase 'leaving money on the table' gets used frequently. I translate that to mean that if they're not fucking you over for every possible cent, they feel like they're not doing things properly.

So - shrug. No idea why but the Li battery prices seem to be dropping here at the electronics places, don't know about marine. I'll go and collect pricing at the Sydney boat show in August. Should be entertaining.

FKT

 

[IStream]

Let's see. A group 31 is 12.9" x 6.75" x 9.27". That is 327 mm x 171 mm x 245 mm.

280 Ah prismatic LFP cells from Eve are 173 x 72 x 208 mm. If we multiply the 72 mm by 4 that brings it up to 288 mm. Looks doable to me. No problem. They probably just put prismatic cells inside.

Fair enough. According to their website, the case dimensions are 338×184×254mm so it looks like they're stretching the case beyond the Group 31 spec by roughly 1/2" in each dimension. Between that and the volumetric efficiency of the prismatic cells, it makes sense.

Also, I mis-remembered and most of the 100-135Ah LiFePO4 cells I was thinking of are actually Group 27, not 31. The editors regret the error.

 

[SVArcturus]

I think if you have one battery with 8 feet of 2/0 and another battery with 2 feet of 2/0 cable, there is a possibility that the battery with only 2 feet of cable will contribute more current during starting. I am definitely not trying to say that 2/0 is inadequate for a starter circuit. It definitely is. I am too lazy to do any real analysis or calculations though. I think with three batteries you are going to be fine.

It's the whole circuit resistance that can be used to provide the control needed to prevent that problem. As long as one "long" cable goes the positive of one battery and the other "long" cable goes to the negative of the other battery, and the jumps between the battery posts of the two batteries are the same length, then one doesn't run into that problem of most of the load (and charge) ending up going to or from one battery. Same with three batteries. One does have to think the cable length arrangement through, though, to prevent the exact problem you mentioned. I had to make one of my "long" cables longer than actually needed by a foot or so to keep the two cables equal length (and equal resistance).

 

[loungesailor]

We went with a single bank without a starter battery over 20 years ago. The system has been reliable and robust for the kind of full time cruising and liveaboarding we do.
Don’t have a jump starter pack in the set up but it does sound like a good idea.

In spite of our positive experiences with a single bank I’m hesitant to recommend the idea to others. Maybe it’ll work and maybe it won’t so finding out would mean taking a chance. Perhaps it’s our particular set up, simple needs, habits and requirements that allows a single bank system to work well.

There’s a company in England that makes non electric spring starters for small diesel engines. When contacted they told me they didn’t make a starter for my model Yanmar but suggested they would if I ordered one from them, maybe someday.