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Old 01-30-2014, 12:17 PM   #1
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Charge Rates

Anyone know the charge rates for our house batts expressed as a "C" number??

Trojan doesn't spec theirs using that number, but the Magnum docs explain it that way.

C5 is a pretty high charge rate, I think. But C20 is maybe more appropriate.

Magnum says to get the proper charge % figure to enter into your control panel, you take the total AH of the stack and divide it by the C number to get the charge rate.

450 divided by 20 is 22.5 amps. With a max charge spec of the Magnum at 70 amps the the percentage would be 22.5 divided by 70 giving a charge % of 32%.

If this is all correct then it's no wonder that my batts were cooked the day I picked up the coach and there was liquid on top of the batts. The default setting on the Magnum is 100%. So they were being charged at 3 times the recommended rate. OUCH!!

And the 100% can't be changed unless you upgrade your panel.

Or maybe it CAN be changed by the installer and FR just forgot to do it when the coach was built?

Boowho??
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Old 01-30-2014, 03:27 PM   #2
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"C' simply stands for capacity.

Let say for an easy example you have a 1 amp hour battery that can only be safely charged at 1C. The charge rate would be 1 amp. If it could be safely charged at 5C you could charge at 5 amps, 10C would be 10 amps, etc. Charging a "dead" battery at 1C will charge it to full capacity in 1 hour.

You said you thought C20 would be more apropriate than C5 but charging at 20C is 4X more current than charging at 5C.

In your example you listed 450. What is 450 representing? If you have a battery bank with 450 amp hours of capacity that's alot of capacity. For the formula to figure out charge rate you need to know the capacity of the battery in amp hours.


UPDATE: I just checked out the manual for an OEM Magnum inverter/controller. You need to be very careful. You indicated C5 and C20 but the manual actually shows C/5 and C/20 which is capacity divided by 5 or 20. This would equate to .2C or .05C charge rate. The manual I see indicates the OEM panel does allow the user to adjust the maximum charge rate but only 3 different rates, 10%, 50%, and 100%. Since your inverter is rated at 70 amps I would recomend the 10% setting so your max charge would be 7 amps ( which is only .015C if your battery bank is 450 amp hours). It will take about 64 hours to fully replenish the battery bank from "dead". The next step would be 50% or 35 amps and fully charge the bank in about 12 hours. This may be OK depending on what batteries you are using. If you post the model of your control panel I may be able to tell you how to adjust the rate.


I hope this helps at least some, Rob

Hopefully someone else who has the same setup as you will chime in as well.
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Old 01-30-2014, 08:20 PM   #3
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Right... It should be c/5 and c/20. 450 is the total amp hours of the stack. Therefore at c/20 its 22.5 amps on the charge rate.

I just got a response from Trojan tech support, they indicate a 10-20 percent rate is acceptable. In other words, 10% of 450 is 45 amps charge and 20 % would be a max charge rate of 90 amps.

I still thing I'll stick with 30% of 70 amps which should make the batts very happy and possibly last a lot longer. Perhaps if I were charging off the genset whilst I was out in the boonies, then I might kick it up a bit higher to keep the genset from running so long.

But charging while attached to shore power, there's no reason to be in a hurry.

Thanks for your response.

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Old 01-30-2014, 08:55 PM   #4
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ratobin, Welcome to the forum, nice post thanks for the info.
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Old 01-30-2014, 09:14 PM   #5
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I profess to be confused. Other then the short bulk charge period, why would the amp rate matter. At all other times the charging is dictated by the voltage not the amperage.. If the batteries are cooking, then the charging "rate" is not the problem, its the voltage.

Look up the 3 or 4 stage charging algorithm, it is only charging dependent during the bulk charge period, which is not very long.

Something else may be going on.
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Old 01-31-2014, 11:43 AM   #6
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Quote:
Originally Posted by garbonz View Post
I profess to be confused. Other then the short bulk charge period, why would the amp rate matter. At all other times the charging is dictated by the voltage not the amperage.. If the batteries are cooking, then the charging "rate" is not the problem, its the voltage.

Look up the 3 or 4 stage charging algorithm, it is only charging dependent during the bulk charge period, which is not very long.

Something else may be going on.
Don't know what to tell ya....... This is the answer I got from Trojan and mention nothing about volts; just amps.

--------------------------------------------------------------------

Below is Trojan Battery's response:

Maximum charge rate is 10-20% of the total capacity. 450Ah (10%) = 45A; (20%) = 90A

If you have additional comments or questions, please contact me at the information below.

Sincerely,

(Name removed)
Technical Support Engineer
Trojan Battery Company

------------------------------------------------------------------

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Old 01-31-2014, 12:46 PM   #7
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That applies to the bulk charge period. They didn't finish the answer. Yes you want to limit the amps during bulk charging. But that does not apply to a battery plugged in over time at float.

TRIGUY, help me here
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Old 01-31-2014, 03:28 PM   #8
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Quote:
Originally Posted by garbonz View Post
That applies to the bulk charge period. They didn't finish the answer. Yes you want to limit the amps during bulk charging. But that does not apply to a battery plugged in over time at float.

TRIGUY, help me here
All I know it that volts multiplied by amps equals watts. But here is additional
info from the inverter control panel manual:



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Old 01-31-2014, 04:56 PM   #9
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I think this graph might explain Trojan's response. (Keep in mind that their reference to the C20 rate in the graph is the Ah capacity at a C/20 discharge current or capacity of the battery at the 20-hour rate of discharge. You might sometimes see other rates but, in general for RV uses, the rated capacity is quoted for a discharge time of 20 hours.)

I understand the tech on the phone said 10-20% of capacity, but their material always has stated 10-13% AFAIK. Their recommended profile for the bulk phase for a flooded battery is 10-13% of C20 for the first 80-90% and then amps drop quickly as voltage rises.

When starting to charge a battery, voltage immediately jumps to about 2.1 V / cell (12.6 V for a 12 V battery) and then slowly rises until the first voltage limit is reached. This is the bulk phase of the charge cycle, during which the battery will accept the full available charge current. Current needs to drop from that point forward.

For those who care for some more detail as to why amps drop so drastically: A deeply discharged battery will accept a current of this order of magnitude until it is about 80% charged. It will then reach the first voltage limit. From there onwards, instead of “absorbing” all of the current being “offered”, charge acceptance reduces rapidly. Therefore this first voltage limit is called the absorption voltage and the subsequent phase of the charge cycle the absorption phase.

A high bulk-charging rate will heat the battery, increase gassing and increase the absorption time needed to fully charge the battery. In other words: a high charging current will only shorten charge time to a limited extent. In any case, the current must be limited because once the gassing voltage has been reached the active mass will be pushed out of the plates due to excessive gassing if there is more current than the battery can absorb. Basically, absorption is a trade off between voltage and time.

Absorption continues until the current has declined to about 1-3 amps and the battery is about 98% charged. At that time, you enter the float phase with a charge that will not boil or heat batteries but will maintain the batteries at 100% readiness and prevent cycling during long term inactivity.
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Old 01-31-2014, 07:47 PM   #10
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I thank you both for the in depth explanations, but as a simple RV'er (as opposed to a battery engineer/designer) I'd rather not know all that.

I sincerely hope that it's not NECESSARY to have this level of knowledge in order to not ruin another battery stack.

My only goal is to NOT cook these new batts as happened to the originals.

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