Emergency starting

[learning all the time]

I just ordered my LifePO4 batteries for my motorhome. I realize these are not starting batteries but I just realiezed that in an emergency I can use my house batteries to start my engine. My questions are 1.) Would these start my batteries if absolutely needed. 2.) Would it damage the batteries if I had to use them as a starter.

 

[FilterGuy]

Unless the starter battery is completely shot, the LiFePO4 does not need to turn over the engine, they just need to top off the starter battery. The small portable starter packs you can buy do this.

I would not try to do it in anything but an emergency, but yes, you can use the house batteries to get the engine started.
However, you should plan ahead for how you would do it.

If you want to use jumper cables, make sure you have a good place to clamp the cables to. (The cell terminals are *not* good places to clamp a jumper cable)

Note, if you use a very heavy gauge wire to permanently tie the starter and house battery negatives together, you only have to jump the positive

I once had a boat owner install a heavy duty DC contactor (relay) between the house and starter battery positive. The coil of the contactor was wired to a momentary contact toggle switch with a safety cover. In an emergency he could hold the switch on for 20 or so seconds and then start his engine. (It was momentary contact to prevent it from accidentally being left in the on position.)

 

[snoobler]

FG, FYI,

In most cases, a motorhome has a relay that can join the coach and chassis battery with a press-and-hold switch allowing both sets of batteries to be used to start the engine or generator. This same relay is often automatically closed to connect both sets while charging them. It also disconnects the chassis battery if total system battery drops below 12.7V (to ensure the chassis battery can still start the engine). Additionally, while driving and producing current from the engine driven alternator, the coach batteries are automatically joined to the chassis battery to charge the coach batteries from the alternator.

He will have no need of jumper cables if his motorhome's BIRD and solenoid are operating correctly. Even if they aren't, they are on a common ground, and the two (+) can be joined with a single jumper cable clamp spanning both contacts.

 

[FilterGuy]

Since the OP mentioned he just ordered his battery, I assumed the house battery is new. However, you are correct, there may be existing house batteries with interconnects.

If there is an existing relay, it needs to be set up to only engage when the button is pressed. You do not want the alternator trying to directly charge the LiFePO4.

 

[John Frum]

In most cases, a motorhome has a relay that can join the coach and chassis battery with a press-and-hold switch allowing both sets of batteries to be used to start the engine or generator. This same relay is often automatically closed to connect both sets while charging them. It also disconnects the chassis battery if total system battery drops below 12.7V (to ensure the chassis battery can still start the engine). Additionally, while driving and producing current from the engine driven alternator, the coach batteries are automatically joined to the chassis battery to charge the coach batteries from the alternator.

Battery isolators imho are more trouble than they are worth with lifepo4 house batteries.
This dc2dc charger will top up your starter battery from solar after the house battery is full.

DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT

Make sure your service battery has all the juice it needs with Renogy's DCC50S DC-DC MPPT On-Board Battery Charger. 10% off: Renogy10off. Free shipping

www.renogy.com

 

[FilterGuy]

Battery isolators imho are more trouble than they are worth with lifepo4 house batteries.
This dc2dc charger will top up your starter battery from solar after the house battery is full.

DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT

Make sure your service battery has all the juice it needs with Renogy's DCC50S DC-DC MPPT On-Board Battery Charger. 10% off: Renogy10off. Free shipping

www.renogy.com

I would even say they are really bad for LiFePO4 batteries. If the OP has one it needs to be removed or, if possible, rewired to to only be engaged manually for emergency situations.

 

[snoobler]

Agree with the trouble. Solenoids fail more often than they should, and you have nothing. It is an all-or-none situation typically. Yank the fuse, and it's disabled in all respects.

I'm probably not up to speed on best practices, but an alternator typically peaks at 14.0V and pulls back very rapidly to mid-13s. My 5th wheel has a simple constant-voltage AC/DC converter that "floats" my DIY LiFePO4 battery at 13.6V, and I don't have any problems with it, but I rarely ask anything of the battery since it's on off-grid "shore power"... Looks like a test is in order.

DCC50S - nifty unit. Aside from the cost, it looks great.

 

[FilterGuy]

If you drive your LiFePO4 with a continuous 13.6 volts and don't let them go to a float mode, you will age them quickly.

Also, if your LiFePO4 bank is near empty, they will take every amp your alternator can generate (and then some). That is a good way to burn out the alternator.

DCC50S - nifty unit. Aside from the cost, it looks great.

Unless you have a really high amp alternator (like 100 or more amps), I recommend 30amp units for casses like this. Bigger is not necessarily better for either the alternator or the battery.

 

[John Frum]

If you drive your LiFePO4 with a continuous 13.6 volts and don't let them go to a float mode, you will age them quickly.

Do you have any reference for this?
I'm planning to make a online ups with lifepo4 and was going to continuously float the 8s battery at between 3.3 and 3.4 volts per.

 

[snoobler]

I did detailed capacity testing of these cells before building the battery, so I'll eventually test them and check the assertion...

 

[FilterGuy]

Do you have any reference for this?
I'm planning to make a online ups with lifepo4 and was going to continuously float the 8s battery at between 3.3 and 3.4 volts per.

Its been a while but as I recall, floating at 3.3-3.4 should be fine. (Ideally, you would turn off the voltage all together)

 

[learning all the time]

FG, FYI,

In most cases, a motorhome has a relay that can join the coach and chassis battery with a press-and-hold switch allowing both sets of batteries to be used to start the engine or generator. This same relay is often automatically closed to connect both sets while charging them. It also disconnects the chassis battery if total system battery drops below 12.7V (to ensure the chassis battery can still start the engine). Additionally, while driving and producing current from the engine driven alternator, the coach batteries are automatically joined to the chassis battery to charge the coach batteries from the alternator.

He will have no need of jumper cables if his motorhome's BIRD and solenoid are operating correctly. Even if they aren't, they are on a common ground, and the two (+) can be joined with a single jumper cable clamp spanning both contacts.

The emergency engine start button is correct in my case. I always carry a battery charger with me that I could plug in but my question is for the extreme scenario case. Thanks for the info.

 

[learning all the time]

Battery isolators imho are more trouble than they are worth with lifepo4 house batteries.
This dc2dc charger will top up your starter battery from solar after the house battery is full.

DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT

Make sure your service battery has all the juice it needs with Renogy's DCC50S DC-DC MPPT On-Board Battery Charger. 10% off: Renogy10off. Free shipping

www.renogy.com

Thanks SmoothJoey. I have been looking into an option like this. I always plug the engine battery in at home and this type of option would make that unneccessary.

 

[learning all the time]

After looking at the dc to dc system I am seriously considering a solar suitcase instead. I am of course upgrading from 200ah of lead acid to 560ah of lithium and currently only have 200 watts of solar. I am looking at 2 Renogy flexible panels for various reasons. So I like the suitcase as I can move it from my house batteries to my starting battery if needed. I can also relocate to more sun when available. Do you guys have experience with these? I'm thinking a renogy 200w suitcase with built in charge controller taking me to 400 total watts when needed.