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Old 12-08-2022, 08:04 AM   #1
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If running lithium why should I have separate house/start batteries?

Is the only reason so I can still start the engine if I drain my house batteries? If so wouldn't having something like a 1/2/all battery switch like I have on my boat solve that issue since i'll have multiple batteries? Or just have my Victron setup to turn off inverter when battery level is under 25%. Or even using the relay on the inverter setup to cut off the 12V power to everything except house when under like 20%.

I can then further optimize my house wiring to be dedicated to required engine/drive components and not interior lights/cigarette sockets, radio and most other things that wouldn't be needed in an emergency... cutting all parasitic drain.
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Old 12-08-2022, 08:32 AM   #2
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I think the prevailing thought from the 30,000 foot level is two-fold -

1) house and coach batteries are generally different and how thief are charged is different, and
2) reduction of risk for being able to start the engine.

From what you state about your boat, I would think you are at a higher risk of getting stranded.
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Old 12-08-2022, 08:40 AM   #3
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If lead acid or AGM they need to be different types because starter needs high amp draw but house needs deep cycle. Lithium does both and would be same voltages/charging rates from my understanding.

I can even setup autostart with my genny so if its under like 50% charge it'll just start and charge providing there's over 1/4 tank of diesel.

I see much less risk of being stranded and I already keep a jump pack in the rig so worst case I can hopefully jump the engine or genny.

Just didn't know if there's other issues i'm not considering by tieing 2 systems together
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Old 12-08-2022, 08:47 AM   #4
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Is the only reason so I can still start the engine if I drain my house batteries?
With lead acid batteries they are different designs between deep cycle and starting batteries. Deep cycle cannot provide the needed burst amperage for large starter motors. However large parallel banks of them could certainly do it like some RV's have, but yes you would want the redundancy and isolation of starter batteries anyway with a manual combine switch in case starter batteries have an issue. Dual purpose batteries are usually the worst of both worlds, some high quality AGM's are deep cycle than can also be used for starting no issue, like say Lifelines.

With lithium it is normally not recommended to use them for engine starting due to the BMS not being able to handle the large surge current. Either the BMS will detect the initial surge as a short circuit and shutdown or potentially cause a BMS FET failure with repeated starting. The other issue is if the BMS shuts down due to say hitting a over voltage/full charge while driving this will cause a large voltage spike from alternator that could cause damage, you need a battery to buffer alternator output, abruptly removing the battery from the system is "bad".

I have used my lithiums to start my gas MH generator which had about a 270 amp surge however on my DP I moved the generator over to chassis batteries for starting as it was 390 amps surge and to me it makes more sense to start generator off chassis anyway in case I run down house. I have a bidirectional DC-DC charger that keeps chase batteries charged from house and vise-versa.

My 6.7L diesel has two large starting batteries and exceeded my clamp meters 1000 amp inrush current limit when starting. Diesels take much more starting current than gas due to high compression and things like glow plugs and grid heaters.
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Old 12-08-2022, 09:01 AM   #5
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With lead acid batteries they are different designs between deep cycle and starting batteries. Deep cycle cannot provide the needed burst amperage for large starter motors. However large parallel banks of them could certainly do it like some RV's have, but yes you would want the redundancy and isolation of starter batteries anyway with a manual combine switch in case starter batteries have an issue. Dual purpose batteries are usually the worst of both worlds, some high quality AGM's are deep cycle than can also be used for starting no issue, like say Lifelines.

With lithium it is normally not recommended to use them for engine starting due to the BMS not being able to handle the large surge current. Either the BMS will detect the initial surge as a short circuit and shutdown or potentially cause a BMS FET failure with repeated starting. The other issue is if the BMS shuts down due to say hitting a over voltage/full charge while driving this will cause a large voltage spike from alternator that could cause damage, you need a battery to buffer alternator output, abruptly removing the battery from the system is "bad".

I have used my lithiums to start my gas MH generator which had about a 270 amp surge however on my DP I moved the generator over to chassis batteries for starting as it was 390 amps surge and to me it makes more sense to start generator off chassis anyway in case I run down house. I have a bidirectional DC-DC charger that keeps chase batteries charged from house and vise-versa.

My 6.7L diesel has two large starting batteries and exceeded my clamp meters 1000 amp inrush current limit when starting. Diesels take much more starting current than gas due to high compression and things like glow plugs and grid heaters.
Thank you!! I'm planning on starting with 2x400AH lithium (EG4 rack ones) then adding another 2 once I 'm happy with them. They're rated for 200a continuous discharge so if 4 in parallel will be rated at 800a. Unsure max discharge though but I'm assuming double would be fine. I have a 5.9 cummins with 2x950 CCA lead acid start currently
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Old 12-08-2022, 09:12 AM   #6
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Thank you!! I'm planning on starting with 2x400AH lithium (EG4 rack ones) then adding another 2 once I 'm happy with them. They're rated for 200a continuous discharge so if 4 in parallel will be rated at 800a. Unsure max discharge though but I'm assuming double would be fine. I have a 5.9 cummins with 2x950 CCA lead acid start currently
I did forget the other issue with charing lithiums directly off the engine alternator, if a large bank like yours is discharged, their low resistance will pull all available current from the alternator until nearly full charge, this potentially maxes the alternator for many hours. Most alternators in MH's are not designed for continuous duty and could cause premature failure. Alternators typically don't have the best voltage profile for lithiums either and will hold them above 14v once charged for the entire trip.

I use a Renege Rego 60 amp DC-DC charger to limit current and do a proper float/absorb voltage from alternator it also charges chassis as needed. The other option is a lithium aware combine relay like the Precision Circuits lithium BIM that turn relay on and off at intervals to reduce duty cycle of alternator.

I would not try to use your lithium bank for starting the 5.9, it may technically have the surge capacity, but could be touchy depending on how sensitive the BMS's are and mainly you don't want the alternator issues with that setup. Again it's not typically the lithium cells that have an issue, they are very stiff and have very high momentary surge ability, its the BMS and alternator that cause the problem.
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Old 12-08-2022, 09:21 AM   #7
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Unsure max discharge though but I'm assuming double would be fine. I have a 5.9 cummins with 2x950 CCA lead acid start currently
Looks like the 12v EG4 have 220 amp for 10 seconds, 250 amp for 3 seconds with a 775 amp for 100ms short circuit. 4 in parallel gives you 4 times that amount which again could technically work, its not easy on those MOSFETS but there might be enough headroom there.

I think the alternator charging situation is still the main issue.
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Old 12-08-2022, 09:38 AM   #8
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Thanks so much still learning. I have this 320a alternator coming in tomorrow and ideally I'd have some DC to DC charger between them but I don't see any high amp chargers and unsure if I can run those in parallel.

I'm wondering if I can just use Victrons BMS 200 on the alternator wiring to limit the alternator, leaving the system + unplugged. But that'll only provide 100amps of charging making the whole point of a larger alternator mute.

https://www.victronenergy.com/batter...art-bms-12-200

https://www.mechman.com/alternators/...-5-9l-cummins/
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Old 12-08-2022, 10:08 AM   #9
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Thanks so much still learning. I have this 320a alternator coming in tomorrow and ideally I'd have some DC to DC charger between them but I don't see any high amp chargers and unsure if I can run those in parallel.

I'm wondering if I can just use Victrons BMS 200 on the alternator wiring to limit the alternator, leaving the system + unplugged. But that'll only provide 100amps of charging making the whole point of a larger alternator mute.
An alternator physically that small is most likely not rated for 320 amps continuous duty, and the lithiums will pull that easily when discharged, also all wiring between alternator and batteries will need to be able to handle 320 amps continuous and most likely need upgrading.

There is not simple way to regulate current from alternator, DC-DC chargers typically max out at 60 amps, I am sure there are bigger ones out there probably from the marine world that cost much more. You can also parallel DC-DC chargers to get more current.

Most setups that are trying avoid using a generator are using dedicated secondary alternators for charging designed for continuous duty and have better voltage regulation for lithium. There are also alternator regulators that can be used such as https://www.wakespeed.com
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Old 12-08-2022, 10:40 AM   #10
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Its so weird there isn't anything to easily limit the voltage and amperage of the alternator. I'd assume the BMS would do this and was the whole point of a BMS. I guess I could go with 4x 70amp converters and call it a day.

All wiring is going to be upgraded to 4/0
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Old 12-08-2022, 10:47 AM   #11
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I'd assume the BMS would do this and was the whole point of a BMS.
BMS is just a on/off protection device, if you are tripping your BMS you have gone too far, it more like a circuit breaker. There are some really new ones that can regulate current in specific situations but still not for this, maybe one day but then they will be basically close to a DC-DC charger in design complexity and cost.

Remember your chassis need a certain amount of amperage as well usually at least 30 amps is a good number but could be more and you want that stable for things like engine ECU's, so you must figure that on top of what you're pulling to charge batteries. Again this is why if you serious about high amperage charging using engine rather than generator a dedicated alternator is typically used.

I would be shooting for 50% continuous load on little alternator like that so 160 amps.
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Old 12-08-2022, 12:11 PM   #12
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I don’t recall how I did it but when I figured out the amp pull from the engine and accessories I was surprised how high it was.
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Old 12-08-2022, 12:29 PM   #13
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I’ve been starting my 5.9 with 4 trojan t-105 8 year old house batteries for several years, no issues.

Even discharged to nearly 50% they will still crank it.
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Old 12-08-2022, 01:27 PM   #14
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I have started my 8.9 Cummins on 400 amps of Lifepo4 batteries without any consequences.
I needed to do this when my chassis batteries had gone flat because shore power charging had failed.
Along with that would have been the load of paralleling both sets of batteries as well.
It all worked as intended. I left my MOM switch/relay in parallel with the DC to DC charger just incase I needed it.
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