Need some advice about LiFePO4 DIY batteries for both engine starting and solar use.

[LarryJForman]

I have some questions about using DIY LiFePO4 batteries for both starting my big block Olds 455 engine as well as running my 1970 Ultravan 22 ft Class A RV. I have been using two old (3-4 years old) Optima 12V AGM batteries that are now weak. I need to test them to see exactly how weak. I would like to migrate to DIY LiFePO4 12V batteries and hope to build one or possibly two 280 AH batteries. At this point I only have 2 100 Watt 12V panels powering a 40 Amp HQST MPPT solar charger and hope to add two more 100 watt panels in the near future. At this point I have not measured the big block engine starting current drain and need to verify that I don't over discharge any new Lithium batteries when starting the engine. I know the weak 12V AGM Optimas are still working OK at this point and able to start the engine reliabily. I wonder if I could just use initially one DIY 280 AH Lithium Iron Phosphate battery for both starting as well as house battery until I build a second battery for house use. I do know that I will need to add an alternator buffer so as to not over heat the alternator when on the road. I wonder if I could initially use maybe a 280 AH 12V DIY LiFePO4 for both engine starting and solar house battery and then build a smaller and lighter 100 AH DIY LiFePO4 battery for engine starting use for the big block engine? I do have a 2500 Watt Onan propane powered generator on board if I get into trouble and have a 70 Amp on board battery charger. I have a 2000 W pure sine wave inverter to power the AC system when on the road and boondocking. I also have a small roof A/C that I was thinking will likely not be able to be powered by a small 400 Watt solar system. Looking for some advice moving towards DIY LiFePO4 12 volt batteries. Since I just joined I will scour the posts and then be able to ask more intelligent questions. Many thanks for any suggestions. -- Larry Forman

 

[newbostonconst]

Good Luck on starting that engine.....guessing around 900 amps to turn it over...Wondering if you feed it 5 cells to keep the voltage up and thus draw less current....

Are you using the factory starter? I think there are gear reduction starters available for them that would also cut down on the amp draw.

 

[LarryJForman]

Good Luck on starting that engine.....guessing around 900 amps to turn it over...Wondering if you feed it 5 cells to keep the voltage up and thus draw less current....

Are you using the factory starter? I think there are gear reduction starters available for them that would also cut down on the amp draw.

I read that the rule of thumb for starting an engine is 1 Amp per cubic inch. Just received a 1000 Amp clamp on meter and measured the starting amps and it was about 400 Amps for my 455 CI engine, so about right and maybe a little low due to warmer weather. I am using a factory starter as a gear reduction one died and they are no longer available for a 1970 engine. I still need to figure out how to start this engine assuming I go with 280 AH as the 1C is 280 Amps and not close enough to 400+ cranking amps. I ordered two more 100 watt panels from HQST, but they are backordered and will not ship until after they receive stock on about 4/28/21. Now wondering where to order the four 280 AH cell with all the Alibaba quality issues people are having. I NEED assurance of receiving four good 280 AH cells and don't know where to go.

 

[Bud Martin]

You also have to think about the BMS.

 

[LarryJForman]

You also have to think about the BMS.

Yes, good point. I have been following what Freely Roaming on Youtube has been doing building his 280 AH battery in a wood box with compression, heating, cooling and active and resistive BMS. Not sure which BMS I will use right now. Also need a DC to DC charger from the alternator to charge the battery when driving. Thinking of 20 amps with the solar might be big enough. Might also need a new AC charger with LiFePO4 profile for when camping with power at RV park, in case solar is not adequate.

 

[newbostonconst]

You also have to think about the BMS.

There isn't a BMS that big that he or I can afford.....

 

[Ampster]

There isn't a BMS that big that he or I can afford.....

There are ways to bypass the size limit of small BMSs to run a contactor. Those solutions are less expensive than full on contactor based BMSs like Orion or others. There is no practical current limit on those solutions. My Kilovac contactor can handle 500 Amps.

 

[mikefitz]

I still need to figure out how to start this engine assuming I go with 280 AH as the 1C is 280 Amps and not close enough to 400+ cranking amps.

Keep lead acid for engine start and alternator protection.

Mike

 

[Ampster]

Keep lead acid for engine start and alternator protection.

That is a good solution for now. If it were me I would invest in four more 280 Ahr cells and work on finding the best alternator workaround for when the lead acid batteries die. There are lots of alternator solutions on this site alone. Resistors, DC to DC converters and long wire runs to name a few.

 

[Bud Martin]

There isn't a BMS that big that he or I can afford.....

Contactor based BMS.