car batt question

[whitmanink]

my friend is getting into helis, and he wants to charge his 6s 5000mah lipos , and his rc boat ( I think 3s lipos) anyways , he is tight on cash , as he just bought a heli and a new radio , he wants to use a car battery and use it for his charger ., now with that said, he has a 850watt d\c charger , heres the fun parts, can it be done? will a single car battery, commited to only use on the charger , not in a car, charge a 6s lipo?
.could it work? could he get any good charge c ratings? how many charges before the battery would need to be charged , if its at all possible ,if it is possible ,than what size battery should he get or recommend ? he plans on getting deepcycles, but not next spring,, thanks guys

[RedOctobyr]

To make sure I understood, he's talking about using a car starter battery, right? And not getting deep-cycles until next spring?

I've charged a group of 6S packs, totaling 4800mAh, off an old car starter battery, using my 250W charger. It definitely works. But starter batteries are not meant for any sort of deep, constant-load discharging. They're meant for short bursts of huge current, then immediately being recharged.

You really want deep-cycles for this application. Or a small generator (if noise, etc, is OK, the cheap $90 800W 2-stroke Harbor Freight generator is an option).

My battery is a retired one from my car. So mistreating it a bit is OK with me. (I have since gotten a generator)

Doing this for several 6S 5000s off a starter battery may be pushing it. But if he already has the battery, and it's not doing anything, sure, see how it goes. Worst-case will be an early death for the car battery. Recharge the car battery as often as possible, and that charge should be fairly slow (overnight).

The most power he'll get depends on the input current limit of his charger. Assume 12V * input current * 0.9 (charger efficiency). If the charger could take 40A input, that would be about 430W output. 430W / 25.2V (peak 6S voltage) = 17A of charging.

Starter batteries are rated by brief output current, not total energy storage. Deep-cycles are rated by total energy storage (Ah), but they use optimistic numbers (based on a really-slow discharge; the available energy drops for fast discharges, like our use).

But you can do this, it's just tough on a starter battery.

[Precipitate]

1000mAh equals 1Ah. If you charge half of that 5000mAh back into that pack, you would be taking 2500mAh or 2.5Ah out of your car battery. If its a 100Ah car battery and you could discharge it completely without damage (you can't by the way and non-deep cycles don't like deep discharge at all) you would be left with 97.5Ah in the car battery. This should give you an idea.

[Precipitate]

A deep-cycle battery is designed to discharge between 45% and 75%.

A starter battery is designed for 2-5% max discharge for longevity.

[RedOctobyr]

Just be mindful of voltage. You can't look at just Ah (amp-hours) when the pack voltage is different than the supply-battery voltage.

Putting 2500mAh into a ~24V 6S battery (to make the #'s easier) requires *5000*mAh from a 12V supply battery.

You really need to think in terms of watt-hours, not amp-hours, as that factors in voltage. Just multiply the Ah times the voltage of whatever it is, to get watt-hours.

Putting 2500mAh into a 25.2V 6S pack is 2.5Ah * 25.2V = 63 watt-hours (Wh).

So pulling 63 watt-hours from a 12V supply battery is 63Wh / 12V = 5.25Ah from your 12V battery.

[Precipitate]

Discharging a fully charged deep-cycle 100Ah battery 75%, while putting 2500mAh back in the 5000mAh packs each cycle, should give you around 30 recharges.

[Precipitate]

Originally Posted by RedOctobyr

Just be mindful of voltage. You can't look at just Ah (amp-hours) when the pack voltage is different than the supply-battery voltage.

Putting 2500mAh into a ~24V 6S battery (to make the #'s easier) requires *5000*mAh from a 12V supply battery.

You really need to think in terms of watt-hours, not amp-hours, as that factors in voltage. Just multiply the Ah times the voltage of whatever it is, to get watt-hours.

Putting 2500mAh into a 25.2V 6S pack is 2.5Ah * 25.2V = 63 watt-hours (Wh).

So pulling 63 watt-hours from a 12V supply battery is 63Wh / 12V = 5.25Ah from your 12V battery.

Good to know as I am learning also. Thank you.

[JonJet]

Originally Posted by Precipitate

1000mAh equals 1Ah. If you charge half of that 5000mAh back into that pack, you would be taking 2500mAh or 2.5Ah out of your car battery. If its a 100Ah car battery and you could discharge it completely without damage (you can't by the way and non-deep cycles don't like deep discharge at all) you would be left with 97.5Ah in the car battery. This should give you an idea.

At a flight-time ratio of 80% usage, that would mean 4000 mAh charged back in...

So 100Ah would give a maximum 25 charges to the R/C batteries...should be good for most flying sessions

No reason you cannot also use your vehicle battery to do the same thing, just every 3rd pack or so...wound increase the recharges to 40 packs

If you can park the vehicle near the flight line and it's not a noise issue, just use the vehicle battery and let it idle-charge about half the time and never worry about available battery power while on the flight line

A half-decent set of 20' jumper cables with the charger-ends changed to lug terminals and connected to a piece of 2x4 with 4" long 3/8" bolts to terminate the cables and attach the charger clamps should provide ample safety and plenty of current

I had a similar setup when I was driving OTR to give 110v power to any point on the rig using an inverter, so I speak from experience

[RedOctobyr]

Originally Posted by JonJet

At a flight-time ratio of 80% usage, that would mean 4000 mAh charged back in...

So 100Ah would give a maximum 25 charges to the R/C batteries...should be good for most flying sessions

4000mAh (80% discharged) into 6S is 101Wh. Pulling that from a 12V battery requires 8.4Ah from the 12V.

A 100Ah 12V deep-cycle, if you're going all the way down to 75% used (I've heard you shouldn't go past 50%) means 75Ah available.

75Ah / 8.4Ah = 9 charges, not 25 charges.

If you obeyed a 50%-max-discharge guideline on the 100Ah deep-cycle, you'd have 50Ah available. Meaning only 6 charges.

As mentioned before, read up further on deep-cycle discharges. They typically (and optimistically) base their capacity ratings on a 1/20 C discharge (100Ah, if discharged slowly over 20 hours). That number drops meaningfully as the discharge current goes up. So maybe your 100Ah battery is effectively 75Ah if using a faster discharge, like you would here (I'm making up a #, as I don't recall the relationship offhand).

[Slyster]

As long as the vehicle is idling you can charge whatever the charger maxes out on at 14v or so (car battery being charged by alternator)

Deep cycle tutorial thread
https://www.helifreak.com/showthread.php?t=374446

[JonJet]

LOL ok, perhaps My math is off...

Which is why I mentioned the other option of using the vehicle battery at the pit area

Many of the locals who go to fun-flys around the tri-state area do as I mentioned, since the vehicle provides the workspace, storage and transportation needs of the equipment...not to mention the adult beverage cooler and shade canopy anchor points

40' to the flight line never seems to be an issue leaving the vehicle running as needed

[JonJet]

Originally Posted by Slyster

As long as the vehicle is idling you can charge whatever the charger maxes out on at 14v or so (car battery being charged by alternator)

Deep cycle tutorial thread
https://www.helifreak.com/showthread.php?t=374446

[whitmanink]

wow guys , that's a lot of insight, a lot of things I didn't even know , now I do , yes I was talking about a car battery , but if you can use a single deepcycle , I don't see why he couldn't use one and be fine all day , I will show him all this when I see him tonight, I have a feeling we are going to wallmart or sears for a marine battery.

[RedOctobyr]

Maybe Slyster will weigh in on brands. I believe there were some that are not great. I thought Walmart deep cycles were in that group.

Sent using Tapatalk.

[whitmanink]

he just called me , saying I can charge a deep cycle battery using my icharger 306b? is this true? how could he know what my charger can do , ive never looked past ir readings and lipo charging ,so its possible I don't know it can , that would be kinda cool , it would make me upset I just got a geny

[RedOctobyr]

Yes, it can charge a lead acid. Most modern LiPo chargers can charge several different battery chemistries.

Sent using Tapatalk.

[whitmanink]

yea I just read through my manual and I saw the pb charging section, so let me make sure im right,,lets say for the sake of saying that, I have a 100ah deep cycle battery , that means I got , 100,000mah correct? so I could get 33 charges out of that using a 6s 5000mah lipo ,( saying I put 3000mah back in the lipo each charge) am I correct ? I know you cant deplete a deep cycle to less than say %30 but im trying to see if my numbers are correct?

[Slyster]

No.. voltages are different and there are losses.

You will get 'around' 6 or 7 charges for a 6s5000mah lipo using one 100Ah deep cycle.

[RedOctobyr]

Originally Posted by whitmanink

yea I just read through my manual and I saw the pb charging section, so let me make sure im right,,lets say for the sake of saying that, I have a 100ah deep cycle battery , that means I got , 100,000mah correct? so I could get 33 charges out of that using a 6s 5000mah lipo ,( saying I put 3000mah back in the lipo each charge) am I correct ?

Please see post #9. Or, even better, what Slyster wrote, since he has actual experience with deep cycles.