I edited the thread title and OP as this has turned into a discussion about options for field batteries, rather than just investigating a single type. In efforts to maintain thread history, I left my original text below.

Original Post:
I've done a search and came up short. I know this has been done but not sure if I am correct as to how.

Can you use larger lipo packs (like 6s) as a field battery to charge smaller packs?

I have (6) 2s-800mah, (4) 3s-1300mah and (4) 2s-2200mah batteries. I am flying park-flyers and starting to get my kids involved. Before, I would charge at home and fly all the packs (whatever size I was flying that day as I would only fly one size since it is relative to my three planes) and head home. But now with the kids, more flights at the field will be necessary.

Onward into my twisted school of thought!

The typical hobby type 7ah-12v gel cell is $27 and would only give me enough Wh to charge the small batteries a couple times. I have three kids and they all need their turn at the wheel, buddy box of course! So that would mean two of these batteries at $60+ for 168Wh. Just one 6s-5000mAh pack for $40+ would give me 111Wh. Plus it would weigh 7lbs less....

Am I thinking about this right? I was also thinking if worse came to worse I can just use the car battery. But I'm not too crazy about being stranded with 3 kids.

What are you going to power with the 6s voltage? 5Ah capacity is still 5Ah.

mw

Hi Mark,
I've been told to work in Watt hours rather than Amp hours when speaking of charging? Is this wrong?

I am trying to see if I can use the larger packs as the chargers power source while at the field. I was thinking a couple light weight 6s packs would hold more energy for the same cost as heavier gel cells.

Evan

Yes use watt hours. I mostly charge at home, then use a 100ah deep cycle to fill in at the field. Some people charge off the car battery too. It's not ideal, but you can probably get away with it with smaller batteries like you're flying. Also consider just buying more smaller batteries.

I've seen lipos running chargers, but usually I only see it at indoor events. Typically some outdoor battery just does double duty as the input battery in the winter.

Some people do fear the incendiary potential of the 6S 5000 lipo but that's a personal thing. Basically you have a lot of ways to get there.

If you use a big lipo to power a charger to charge smaller lipos you run the risk of deep-discharging the big lipo and damaging it. As expensive as a car battery.
A battery fuel gauge may be the answer
http://batteryuniversity.com/learn/a...ery_fuel_gauge
http://www.batterymart.com/p-bfg12v-...Fal7QgodKE8ALQ
http://www.cabelas.com/catalog/produ...id=15291837018
http://www.batterystuff.com/battery-...LOWBAT-XL.html

Or any Recreational Vehicle parts store

The general approach is correct. It is better to recharge often than to buy lots of small packs.

But a LiPo is not the ideal field battery, as the calendar life is not very long.

I prefer LiFePo, and thinking in bigger dimensions.

This is my latest project. The case is still unfinished.

5s3p 8Ah Headway cells.

Originally Posted by Mark Wood

What are you going to power with the 6s voltage? 5Ah capacity is still 5Ah.

mw

You can charge two 3S 5Ah packs with one 6S 5Ah pack (in theory).

I don't see the point in investing in a very expensive LiPo to charge a bunch of cheap LiPos, better get a lead battery, they are cheap and the weight does not matter, since it's not leavinig the ground anyway.

A 7Ah lead SLA will cost you $50, a 7Ah 12V LiPo is pushing a hundred bucks.

Originally Posted by Julez

The general approach is correct. It is better to recharge often than to buy lots of small packs.

But a LiPo is not the ideal field battery, as the calendar life is not very long.

I prefer LiFePo, and thinking in bigger dimensions.

This is my latest project. The case is still unfinished.

5s3p 8Ah Headway cells.

Nice! Where's the best place to get those Headway cells? A few places I checked and they seem to run about $15 each.

All good stuff. Thanks!

As far as the lipo cost, I was thinking a low 15-20c 6s-5000mah would suffice and it would fit in the charging case I built. But perhaps I should just redo the case and put in some gel cells as recommended. Simple and safe. Then I can keep the lipos I have in a fire proof container as I am doing now. I forgot the 106b charger can do Pb batteries too. I was trying to avoid needing another charger for the different battery type.

Originally Posted by Ohmic

Nice! Where's the best place to get those Headway cells? A few places I checked and they seem to run about $15 each.

I got mine from www.LiPoPower.de for around €18 each.
$15 is an awesome price!

I have also built a field battery this year.
It is a 16Ah 8s Headyway battery. It works great.

I use a lipo to power my charger for small 1S cells sometimes. Not a bad use for old tired lipos, but I wouldn't go buying any for the purpose, just not cost effective.

My question is what are you going to power the 6s with? A charger?

mw

I was going to power the charger with it when at the field. When I'm home I would recharge it with my iCharger.

But now I'm curious about the LiFePo cells here. In my current charging case I have room for 6-8 8Ah cells, along with the charger and the ac power source. It'd be nice to keep the ac and dc power sources all in one case.

Can i also user this:
http://www.aliexpress.com/item/54000...485928305.html

I need a portable solution that i can pack in a backpack that is not too heavy....