Simple explanation AH vs. V

Re: Simple explanation AH vs. V

adam1984 wrote: »

Think I got it:
1) The bank is 12V and 100AH. Does this mean I have 1200wh available? Also, does this mean at 50% discharge i have 600wh available?

Yep.

2) So we have 1200wh available. I have a fridge that draws 10A off the 120V inverter. What other information do I need to find out how long the fridge will run until the bank is 50% discharged?

Where was that 10 amps measured?

• 10a * 12 volts = 120 watts
• 10a * 120 volts * PF? = up to 1,200 watts

If that is 10 amps at 12 volts DC... Then, at first look 100 AH battery (second look is next answer):

• 100 AH / 10 A = 10 hours to dead (not good thing to do to your battery)

3) What does the 20 hr. discharge rate tell me? When you buy a battery and it says 100AH, is that the 20 hr. discharge rate? Meaning it is delivering 5amps per hour? And if so does that mean that drawing 20Amps will last 5 hours, or is it not linear?

Yes to first question (5 amps for 20 hours = 100 AH to dead).

Not really to the second question... Basically, the faster you draw current out of a battery, the less efficient it is at doing it. The "apparent" amp*hour capacity goes down as the current draw goes up.

I like to use the 20 hour rate as it is realistic for our uses... Say 3 days of no sun and you use the system 8 hours per night... 3 days * 8 hours = 24 hour discharge rate... Of course, we recommend to discharge only to 50% -- so the "true discharge rate" would be 48 hour discharge rate... But because loads are cycled (sometimes more power is taken, some times less), and the charging time is usually limited (sun light, generator time)--we end up, many times, charging at a higher than C/20 rate (which is also less efficient)... So--to average things out--C/20 rate is a useful average.

C/10 is about the rough maximum efficient charge/discharge rate (13% max charge rate is C/7.8 -- close enough for solar work). When you get to C/10 (20%) or higher rates--then you can start heating the battery--wasted energy and possible long term damage if not controlled.

So--the C/XX is not linear, and changes depending on the design of the battery (high current or low current design, etc.). For more information, look up Peukert Effect.

From the NAWS Battery FAQ--you can see for various batteries:

[FONT=Fixedsys]
Battery Type      100 hour rate      20 hour rate      8
==========================================================
Trojan T-105        250 AH           225 AH          n/a
US Battery 2200     n/a              225 AH        181 AH
Concorde PVX-6220   255 AH           221 AH        183 AH
Surrette S-460 (L-16) 429 AH         344 AH        282 AH
Trojan L-16         400 AH           360 AH          n/a
Surrette CS-25-P    974 AH           779 AH        639 AH[/FONT]

Because we recommend that people charge at a minimum of 5% -- that is C/20 -- So, C/20 just comes up a lot and is a good conservative estimate of useful battery energy...

Now--to make your head spin... Many of the loads now-a-days are driven by a DC to AC inverter... So, while the inverter may have started with the battery near 13 volts, the voltage eventually goes down to 10.5 volts (near dead battery + wiring losses)--You end up with not a constant Amp draw--But a constant power draw. As the battery voltage drops, the current draw actually increases. You now are starting to see UPS batteries rated in Constant Power draw in addition to the standard constant current draw.

• 150 watt load * 1/13 volts * 1/0,85 inv eff = 13.6 amps
• 150 watt load * 1/10.5 volts * 1/0,85 inv eff = 16.8 amps

So--another detail that pulls more current than would have been assumed... And leads to another derating factor of an off-grid system (if you want to get into the details).

THANK YOU FOR YOUR PATIENCE. People have been getting upset with me telling me to look it up online. I apologize but I do try to do the best research i can before asking, but I was under the assumption that's what forums are for, to ask questions you can't find the answer to.

I hope that is not happening to you here--We will never learn the answers until we first ask the questions. And it is hard to ask questions when you don't even know what questions to ask.

-Bill

Re: Simple explanation AH vs. V

adam1984 wrote: »

Can you explain in a little more detail the C/20 charge rate? So lets say I have the trojan t-105 with the 20hr capacity of 225AH.
1) The charge rate is 225/20, or 11.25A per hour? Is that correct?

Note that C/20:

• C/20 = C * 1/20 = C * 0.05 = C * 5% -- our "minimum recommended charging rate"

2) And a rate of C/13= 225/13=17.3 Amps per hour? So is this a good rule for charging only or a good rate for discharging as well?

Note that the "13" we use around here is 13% for maximum generic flooded cell battery charging rate:

• C * 0.13 = 225 AH * 0.13 = 29.25 Amps maximum generic charging rate

A "C/13" rate would be:

• C * 1/13 = C * 0.077 = C * 7.7% rate of charge--just a "middle of the road charging rate

A C*0.13 rate is basically:

• C*0.13 = C * 1/7.7 ~ C * 1/8 = C/8 which is a very high rate used for forklifts (recharge in 8 hour). At this rate and above--they may have cooling fans in the battery pack.

A Concord AGM could actually charge at:

• C/0.25 = C*4 = C4 rate (yes, 15 minutes to recharge battery bank

Of course, you will probably cook your cabling and charger trying for this rate...

3) Does this mean charging a 12v 225AH bank at a C/20 rate which is 11.25Amps per hour mean it would take 20 hours from empty with an 11.25Amp charger to have 100% SOC (assuming they made one)?

In basic math terms--yes.

In real terms, you have absorb mode (when the battery is at the 14.5 volt charging and current acceptance of the battery is less than maximum charge current can output (charger is now in Voltage Mode control--vs Curent Mode control earlier when the battery would accept more current than the charger is capable of outputting.).

The Absorb mode is both slower charging and less efficient charging as some water is being electrolyzed into hydrogen and oxygen gasses. That last part requires ~10% or of battery capacity to finish off the charging (why it takes 110-120% energy to recharge a battery instead of 100%--also there are some other losses, like heating and such that reduce the efficiency of discharging and recharging a battery).

4) Are you much more confused than you were two minutes ago before you read this?

We spent most of our school years learning the ideal way things should work. From physics, to chemistry, to government, to history... We spend the rest of our lives learning that none of it is true.

-Bill

Re: Simple explanation AH vs. V

Years and years of making mistakes.

The schooling is fine. It gives you the tools to understand the basics--and then look beyond them to understand the real world implications.

The school of hard knocks period of education can really be seen in the trades. Machining, electricians, carpentry, HVAC, Power, etc... Everything that is passed on through the (now pretty much gone) apprenticeship programs was to provide experience to the young master to be.

Now you can see why a "new engineer" can get razzed when the walk out on the floor/job sites the first time. "Book Learning" gets a few laughs.

But--if you can bite your tongue (yes--you did go through 4 years of heck to get your technical degree) and listen to the people around you--the education you will continue to receive will be used throughout your lifetime.

I spent as much time on our assembly floor as I could (mini-mainframe computers). I listened and helped--and my coworkers on the floor where always more than happy to answer questions and show me where mistakes where made in design. I was about the only engineer that went out to help implement ECO's (Engineering Change Orders)--Whether I was the one that screwed up or just was fixing things that others had ignored for months or years. We all benefited from the exchange.

In the US--Design engineers tended to come directly from school and go up through the design engineering food chain. A stint through Manufacturing Engineering was considered to be punishment/dead end job. In Japan, all the good designers looked forward to their turn through manufacturing (it was a it was part of the promotion/career path as I understood).

When you get out--take any advantages for practical experience you can... It will be very rewarding. And listen to everyone.

-Bill