Combining house and starting battery banks, possible?

[mstumpf123]

Would like to double the amount of lithium batteries i currently have. but, i have no room in Class A battery compartment unless i use the space the starting batteries use. the system is all 12v, currently have 400ah, plus additional starting batts. I am thinking that i can replace the starting batts with another 400ah, but, i am concerned about burning out the alternator. i could add a dc to dc battery charger between the alternator and the batts but i dont think that will allow the batts to feed the starter, as there is only one cable going from the battery to the engine. any thoughts, I understand the downsides of running down the starter batteries, but was curious if you all think it is possible.

 

[HRTKD]

How many amps does the starter on your engine pull? That is the very first question you need to answer.

One of the ratings for batteries is Cold Cranking Amps (CCA). If your RV requires 800 CCA then batteries and BMS that can provide 400 amps isn't going to cut it.

This is the reason that I don't have my Onan 5500w on-board generator connected to my LiFePO4 battery bank which is rated for 240 amps. I think my battery bank is close to being able to provide the amps the starter requires, but at this point I'm not prepared to eliminate the separate 600 CCA starter battery for the generator.

 

[mstumpf123]

it is a diesel pusher, current battery is 1100CCA, pretty sure it uses most of that.

 

[HRTKD]

It's going to take a lot more LiFePO4 and BMS than you have planned to start that diesel. You don't need to worry about the alternator because you'll never get it started!

Easiest solution is to keep the two systems (mostly) separate. A DC-DC charger between the alternator and the LiFePO4 battery bank will protect the alternator and the LiFePO4 batteries.

 

[Rocketman]

The BMS and contractor are one of the main issues. You will need to look at using a contractor rated for 1000amps. I will be using a kilovolt contractor at 500 amps @ 12v for my house batteries - but I haven’t seen any for a 1000 amps. unless you choose to connect the starter directly to the battery- bypassing the Bms contractor. (Think through the problems there)


To protect the alternator- you will need to replace the regulator with one that also checks for temperature of the alternator. - Look in the marine market - I remember someone has one (Balmer???).

I would think you would need a 1000ah @12v lithium battery to do it on the starting side alone. I think you would be looking at a 6p4s w/ the 270/280ah cells.

Good Luck on your research!

 

[mstumpf123]

ok, understood, how about an alternative. attached is a crude drawing of the layout. i have room to add another bank in the compartment with the inverter, but if i do would i need another bmv to measure usage? i have both a pos and negative distribution bar in the compartment as well. it is also where the Charge controller is located. also the cables connecting the house bats to the inverter are routed in a closed channel that is inaccessible, so i cant just add another couple of cables.

 

[HaldorEE]

it is a diesel pusher, current battery is 1100CCA, pretty sure it uses most of that.

Mine E350 diesel uses a pair of 750CCA batteries.

No way would I use lithium for a starter battery.

 

[the_colorist]

I recommend anyone looking at switching to LiFePo4 in general to read this. Yes, it's long but he has some extremely valuable information from the testing they have done and extensive research. Could save someone $$ down the road. Pay special attention to the sections on the alternator and associated protections, charging etc.

LiFePO4 Batteries On Boats

Here is a quote:

"Stop worrying about not being able to start your motor with a low battery. I purposely started our Westerbeke 44HP diesel at the 0% SoC point of 2.9VPC and did so 12 times in a row. After a dozen starts I simply gave up because I got bored. The battery could have delivered many more starts even at 0% SoC. Enjoy the peace and quiet sail back to the mooring or marina at the end of your cruise."

LiFePo4 can handle an incredible high surge, even almost dead. An installer that I know has done some of the same testing as is mentioned in the article. Starting large diesel trucks on 100AH or smaller cells if I remember correctly. They handled it like no lead battery could dream of.

I wouldn't normally recommend this BUT there are situations where that it MAY not be an issue to pull a tap from the cells directly (bypassing the
BMS) and go to the starter ONLY. It would still be prudent to use a contactor between the cells and the starter. You'll want to wire it so that if the BMS sheds, the contactor opens.

Also remember you don't need a contactor rated at 1000A continuous. You are only starting, not continuously cranking the engine. Most high-end contactors will have a surge capacity over time chart. I believe some of the Kilovac 400A or 500A have a surge of 3000A or more for a few moments.

 

[HaldorEE]

I recommend anyone looking at switching to LiFePo4 in general to read this. Yes, it's long but he has some extremely valuable information from the testing they have done and extensive research. Could save someone $$ down the road. Pay special attention to the sections on the alternator and associated protections, charging etc.

LiFePO4 Batteries On Boats

Here is a quote:

"Stop worrying about not being able to start your motor with a low battery. I purposely started our Westerbeke 44HP diesel at the 0% SoC point of 2.9VPC and did so 12 times in a row. After a dozen starts I simply gave up because I got bored. The battery could have delivered many more starts even at 0% SoC. Enjoy the peace and quiet sail back to the mooring or marina at the end of your cruise."

LiFePo4 can handle an incredible high surge, even almost dead. An installer than I know has done some of the same testing as is mentioned in the article. Starting large diesel trucks on 100AH or smaller cells if I remember correctly. They handled it like no lead battery could dream of.

I wouldn't normally recommend this BUT there are situations where that it MAY not be an issue to pull a tap from the cells directly (bypassing the
BMS) and go to the starter ONLY. It would still be prudent to use a contactor between the cells and the starter. You'll want to wire it so that if the BMS sheds, the contactor opens.

Also remember you don't need a contactor rated at 1000A continuous. You are only starting, not continuously cranking the engine. Most high-end contactors will have a surge capacity over time chart. I believe some of the Kilovac 400A or 500A have a surge of 3000A or more for a few moments.

How many Daly BMS would you have to parallel to pass 1000A?

 

[Rocketman]

the_Colorist - OK I learned something from your comment. The Contractor I will be using on my system is Tyco EV200AAANA - I thought it was a 500Amp Relay/Contractor - but it is a 500+ Amp Relay. The spec sheet shows it working at over 4000 Amps @ 28v (For breaking a load only - 650A for making or breaking the load).

Daly BMS's are the wrong type of BMS's to use - I will be using a Batrium BMS - REC-BMS and Orion Jr I think would also work in this situation. You want the BMS to operate a Contractor (Relay) for the switching. I would be tempted to try to start my Diesel motor with my (to be built this spring) new lithium battery - except I have a 400Amp Fuse - I am hopeful that if I ever need to "Battery Boost" my Chassis Battery the 400A Fuse will hold - (At $40 each they are not cheap & I will be carrying a spare).

 

[Rocketman]

mstumpf123 - Your alternative idea may work. What you are proposing is having 2 different 12v lithium batteries on your system. You would need a BMS for each battery. Also because your BMV-712 would not be connected to the second battery another one may be needed on that battery. All the safety stuff would need to be duplicated: Cut-Off switch, Fuses, Battery Monitor.

 

[learning all the time]

Would like to double the amount of lithium batteries i currently have. but, i have no room in Class A battery compartment unless i use the space the starting batteries use. the system is all 12v, currently have 400ah, plus additional starting batts. I am thinking that i can replace the starting batts with another 400ah, but, i am concerned about burning out the alternator. i could add a dc to dc battery charger between the alternator and the batts but i dont think that will allow the batts to feed the starter, as there is only one cable going from the battery to the engine. any thoughts, I understand the downsides of running down the starter batteries, but was curious if you all think it is possible.

One alternative to additional space if you never use your generator you can pull it out and redesign that space for a large LifePo4 bank. Leave your starter batttery alone and you may be much happier. Only time I use my Onan 4000 is for monthly test runs. Lol