Hi Guys

I'm thing of picking up an optima yellow top battery for charging lipos in the field.

I believe it's a 50AH What is the formula to see how many 3cell 11.1 volt 2200 mah batteries I could charge before taking the reserve in the optima to low

Thanks

Gary

Ok I think I might have it please correct me if i'm wrong
50Ah would equal 50,000 MAH devide that by 2200 the size of the battery I'm charging that come to 22 charged batteries but that would completly discharge the optima which you can't do so what is a safe number if my above math is correct? I guess I need to concider to that the 2200's would not be totally discharged either....SEE whyI'm confused

IMO, half your estimate to be easier on your pb battery.
At 12.25 volts your Optima is sitting at 50% charge.
I hope everydayflyer chimes in here, he is the expert on this stuff.
Here is some numbers I had on my hard drive.

100% 1.260 (auto) to 1.280 (industrial) 12.60-12.75
95% 1.255 12.60-12.70
90% 1.250 (Resting S.G. for standard RV battery.) 12.60-12.65
85% 1.245 (Same as above. No point being too picky.) 12.60
80% 1.235-1.240 (We try not to discharge below this point.) 12.50-12.55
75% 1.225-1.230 (1.230=minimum SG for a charged battery.) 12.50
70% 1.220 (Anything below 1.220 is "poorly" charged.) 12.45
65% 1.215 12.40
60% 1.205 12.35
55% 1.200 12.30
50% 1.190-1.195 (Try to never discharge below this point.) 12.25
45% 1.185 12.20
40% 1.180 12.15-12.20
25% 1.160-1.170 (Dangerously low; battery being damaged.) 12.10-12.15
20% 1.150 (Cells die soon at this point. Bye-bye battery.) 11.80-12.00

Rick

Impossible to predict without knowing the voltage of your 2200 mAh batteries.

I try not to go below 50% on a deep cycle battery, but that's just me.

Originally Posted by SteveM732

Impossible to predict without knowing the voltage of your 2200 mAh batteries.

I try not to go below 50% on a deep cycle battery, but that's just me.

OOPS sorry 11.1 volts

Gary

Originally Posted by rampman

IMO, half your estimate to be easier on your pb battery.
At 12.25 volts your Optima is sitting at 50% charge.
I hope everydayflyer chimes in here, he is the expert on this stuff.
Here is some numbers I had on my hard drive.

100% 1.260 (auto) to 1.280 (industrial) 12.60-12.75
95% 1.255 12.60-12.70
90% 1.250 (Resting S.G. for standard RV battery.) 12.60-12.65
85% 1.245 (Same as above. No point being too picky.) 12.60
80% 1.235-1.240 (We try not to discharge below this point.) 12.50-12.55
75% 1.225-1.230 (1.230=minimum SG for a charged battery.) 12.50
70% 1.220 (Anything below 1.220 is "poorly" charged.) 12.45
65% 1.215 12.40
60% 1.205 12.35
55% 1.200 12.30
50% 1.190-1.195 (Try to never discharge below this point.) 12.25
45% 1.185 12.20
40% 1.180 12.15-12.20
25% 1.160-1.170 (Dangerously low; battery being damaged.) 12.10-12.15
20% 1.150 (Cells die soon at this point. Bye-bye battery.) 11.80-12.00

Rick

Thank you Rick very helpful I printed your chart out

Gary

All the above is good stuff! My two cents worth:

Automobile batteries are not designed for deep discharging so, do not discharge an automobile battery below 75% of capacity (you can only use 25% of the available capacity). Letting an automobile battery dischage beyond 50% and 75% of capacity will shorten its lifespan.

Deep cycle batteries can be discharged to 50% of capacity (you can use 50% of available capacity). These batteries are designed/constructed differently from automobile batteries and they can handle the abuse of a 50% discharge.

The chart listed in an earlier post is a great reference, make a copy of it and bring it with you when you fly. Regardless of how many charges you think you may get, you can still check your charging batteries voltage with a multi meter at any time and know when it is time to stop using that battery!

Regarding LiPo's, figure that you use about 70% of capaciity per flight. I use watt/hours when figuring capacity requirements or needs. Also equipment does not work at 100% efficiency. Amps X Volts = watts and mAh (capacity) X V (pack voltage @ 3.7V/cell) = Wh or Watt Hours. Many lipo's now list this number on the pack label. I usually figure lipo Wh at 4.2V per cell and the Auto or Deep Cycle battery Wh at 12V to take in account efficiency losses.

Using an Automotive 12V battery of 50 Ah capacity, 50 X 12 X .25 (25% of capacity) = 150 Watthours of energy for charging available.

2200mAh 3 cell lipo, 2.2 X 4.2 X 3 X .7 (70% of capacity) = 19.4 Watthours of energy needed to charge a 2200 3S1P lipo at 70%DOD (Depth of Discharge)

150 watthours / 19.4 watthours = 7.73 or you could charge your lipo about 8 times.

If you had a 50 Ah Deep cycle battery, you could charge your lipo battery about 16 times.

Using your earlier number 50Ah/2.2Ah = 22 is not wrong, just not real world results.

Michael (if your not fryin, your not tryin!)

Thank you so much Elecfryer

Great explaination of it all. I did print out Rampman's chart and your entire post and sealed them in plastic to take to the flying field.

I just had major cancer surgury in Jan and I'm doing chemo for 6 months flying a rc plane has always been something I really wanted to do sooo BUCKET LIST

I'm getting all my ducks lined up have the plane , charger batts ect> I can sport fly an align 450 Helo but planes have always been my desire so for now doing alot of sim time till the chemo is done

Gary

Glad to be helpfull Gary! Wow, talk about putting things into perspective! I wish you all the best with your recovery and hope you have a geat time with all your R/C models! There are a lot of great people on RC Groups and I for one look forward to helping you with any questions you may have in the future!

Michael (if your not fryin, your not tryin!)

I'm thing of picking up an optima yellow top battery for charging lipos in the field.

One great feacture of that vbatery is a 36 month non pro rated replacement warranty. I purchased on Feb. of last year rto use inside for LiPoy cycling with my Power Lab chargers which have regenerative discharge abilities up to 40A discharge rate.This was one of a few that can stand being charged that fast and had the best replacement warranty. I purchased mine on line from Advance Auto parts using a $50 discount cupon.

Charles

Due to various inefficiencies in charging and running the charger itself, I calculate the following way.

50Ah x 12 volts x 50% = 300 WH

To charge one 3S 2200 pack - 12.6 x 2.2 = 27.7 WH

300/27.7 = 10.8 packs

So 11 packs per full charge would be safe for that setup.

Remember, in between charges, you are probably leaving the charger on. The battery capacity does down over time, etc.