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Old 08-23-2021, 07:53 PM   #1
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Can you use a Li booster battery on a Li house battery?

We picked up our SMB 2020 MB Sprinter 2500 Diesel from SMB North in July. We camped at camp grounds on the way home (3 day drive). No problems, until one morning we found that we drained the LiPO4 house batteries and could not lower the penthouse top. The campground manager brought over a Li jump starter pack and hooked it up, and let it charge our batter for a few minutes until we had sufficient charge to lower our PH top.


Someone told me you should not do this, i.e., boost a Li battery from a Li battery?


Is that correct? We are not drawing high amps, just getting a few volts into our house battery. We are looking at a battery booster with 2000 mA and is rated for jumping a 6.0 L diesel.


Thanks!


Mike
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Old 08-24-2021, 08:29 AM   #2
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Originally Posted by MikeChapelHill View Post

Someone told me you should not do this, i.e., boost a Li battery from a Li battery?

Is that correct? We are not drawing high amps, just getting a few volts into our house battery. We are looking at a battery booster with 2000 mA and is rated for jumping a 6.0 L diesel.

Thanks!


Mike

I'm not a Lithium battery expert but I can't imagine that connecting a Li booster for a short period of time to get your roof down will hurt anything. Your dead battery is actually drawing high amps from the booster when first connected due to it's depleted state. The booster you are mentioning above is likely rated at 2000A not 2000 ma.


You don't want to connect a charger or other device to a Li battery for a long time that's not designed for a Li battery mainly due to overcharging/overheating the battery which could result in a battery fire that is very difficult to extinguish. That being said Lithium batteries used in vehicles have fairly sophisticated battery management systems built in to protect them from thermal runaway when mistreated etc.
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Old 08-24-2021, 03:52 PM   #3
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Boywonder,
Thank you for replying. And yes, it is 2000Amps.

Mike
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Old 08-25-2021, 09:45 AM   #4
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Out of curiosity, does your setup not include charging via the alternator, i.e. couldn't you have just started the engine to lower your pop-top?

If you've been driving -- and charging -- every day before camp, and still managed to deplete the battery overnight, it's probably worth doing the math and figuring out where your energy is being spent and what your limits are. Most likely, you were running a microwave, coffee maker, or induction cooktop, which are very high short-term consumers. Calculate, based on the wattage, how much you can use those before having to worry about running out. Also check if you have any smaller long-term energy drains you can reduce: turning off the inverter when not needed is huge, but it may also be worthwhile to dial down the refrigerator's coldness overnight, which also helps you sleep.
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Old 08-25-2021, 05:18 PM   #5
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You did no harm to your Li Battery. Inside they all have a BMS which protects the battery from overcharging and overdischarging. You may have actually bottomed your battery and tripped the BMS on it and it will not discharge until it gets a charge and goes back above a certain voltage.



Li batteries have different charging strategies but can tolerate normal charging, especially in a pinch. Biggest issue with Li is you should not charge them directly from your alternator. This is because the Li can open up like a sponge and will take as many amps as it can. Typically the BMS will allow a portion or all of the Amp hour rating. Not sure how big your battery is, but some can draw hundreds of amps with the BMS which will toast your alternator eventually with the heavy demand



I am installing a DIY Li house battery in my van. I have a DC to DC converter which will be charging the Li battery from my alternator. It limits current to 15 Amps and will shut itself off when the alternator is not on.



From dead battery you are in Bulk charging anyway, which means that the battery will essentially adjust it's own voltage while drawing amps. Where lithium get touchy is at the top end where chargers switch to constant voltage (14 to 14.5 depending) and then take smaller and smaller currents until topped off completely. BMS will also protect here as you can feed them higher and higher voltages until they swell up and blow. ALL 12V Li batteries have a BMS (battery management system). If you cracked open your battery box, you would see the BMS and four cells connected in series.


From folks who know, I am told that you can alter your charge profile and minimum cutoff voltage on the BMS to get more cycles from the battery. My BMS cuts out at 12V instead of 10V at the bottom. I then charge to 14.1 volts instead of 14.5



That's all I know, but you did no damage.



Edit: and a couple more things because it's a knowledge kind of day and anyone with a lithium should understand them. There are three basic things that can cause an Li battery to become dangerous. Overcharging, overdischarging, and physical damage. A good BMS will protect against the charge and discharge conditions. A good BMS should also protect against charging below freezing, which simply wrecks the cells.



Ok that's really all I got. Happy trails.
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Old 08-25-2021, 10:17 PM   #6
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Great info Flux!
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Old 08-27-2021, 04:57 PM   #7
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Flux,
Thank you, good information. and I am reassured!


Andrew,
Being new to the technology, it was our maiden voyage, there were several issues at play. We have a 2nd alternator, but we parked for over 36 hrs, without starting up. Yes, we did use the induction cook top for short periods of time. But, where we went wrong was using a small fan to keep us cool overnight, and too much shade,


Mike
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Old 08-27-2021, 06:13 PM   #8
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Another thing......LOL......


I spent a lot of time reading up on diysolarforum.com as I was building my DIY lithium battery pack. Lithium is awesome and has come way down in price. For vanning?? Amazing. So much power in a light package.



Another issue with Lithium is that voltage does not tell the capacity story like it does with SLA batteries. Lithium has a really flat voltage vs discharge until it gets to the end, then it falls off a cliff!! You charge a lithium up to like 14.2 volts say and that is full capacity. Then the battery settles in at about 13.5 and stays over 13 for a long long time until it has about 15% left and then starts dropping fast. Charging is the same. If you are charging it seems like forever then all of the sudden the voltage rockets up over 14.



I highly suggest, if you don't have one, a battery monitor with a shunt. A 40 dollar setup and you will be pretty accurate on your capacity.
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Old 08-28-2021, 08:28 AM   #9
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But, where we went wrong was using a small fan to keep us cool overnight, and too much shade,

Mike

Is your small fan 12V or 110VAC?
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Old 08-28-2021, 10:01 AM   #10
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Protect against charging below freezing @ 32F and charging when over about 115F.
The upper limit is not talked about much but it is pretty easy to have interior temps reach 115F.
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