# C/8, C/10, C/20 Battery Charging Rate???

Posts: 3Registered Users
I am trying to determine the "best" charging rate for my battery bank. I have read through a lot of threads on this forum as well as several FAQs on various websites. One point that seems to be universally accepted is that a current equal to 10-13% of the battery bank's Ah capacity is an optimum rate. I couldn't leave well enough alone and just go with this new found knowledge, I had to keep reading...

My battery bank is going to be 4 Trojan T-105s connected in series and parallel for 12V so I downloaded the Trojan User's Guide to get their recommendation. The chart below was copied from that guide. What confuses me is on the right hand side of the graph where it indicates "10-13% C20". My understanding is that Cx = Battery bank Ah capacity/x. My interpretation of the chart is 10-13% of C20 which in the case of the battery bank I will have is (450/20)* .1 to .13= 2.25 to 2.925. This seems WAY too small. Any thoughts on what Trojan is trying to indicate in the graph?

• Posts: 17,615Banned ✭✭
Re: C/8, C/10, C/20 Battery Charging Rate???

I think you're misinterpreting the info.

450 Amp hours / 20 = 22.5 Amps peak charge current. That's the 5% minimum recommended rate.
450 Amp hours / 10 = 45 Amps peak charge current. That's the 10% "ideal" peak charge rate.

The twin line graph shows the change in charge current and cell Voltage in respect to state of charge.
• Posts: 2,680Solar Expert ✭✭✭✭
Re: C/8, C/10, C/20 Battery Charging Rate???

When I saw this chart on the Trojan site, my intrepretation is that it is saying 10-13% of the 20 hour C (Capacity).

Vic
Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
• Posts: 3Registered Users
Re: C/8, C/10, C/20 Battery Charging Rate???

Thanks to both of you. I think that Vic has pointed out where I made my mistake: C20 is shorthand for "capacity when subjected to a 20 hour discharge".
• Posts: 1,280Solar Expert ✭✭
Re: C/8, C/10, C/20 Battery Charging Rate???

In the graph 'C20' represents the AH capacity at a C/20 discharge current.

T105's AH ratings yield 225 AH's at a constant discharge rate of 11.5 amps for 20 hours.

They yield about 180 AH's at a constant discharge of 22.5 amps for 8 hours.

The curve represents a stage 1 (bulk) charge rate of 12% C20 rating or 27 amps.

When the voltage rises to 7.05 vdc per battery it is held at constant voltage, stage 2 (absorb), and charge current begins to taper off.

Normally when the current tapers off to 1-3% of C20 rate (4.5 amps) the charge would be terminated and charger would go to 3rd stage (float) voltage.

The curve shown takes it higher in voltage, up to equalize levels. I would not do this on a regular basis as it is hard on the batteries.