What is the best charging rate for batteries

[robtribble]

I have several set of batteries, all setup for 48 volts, I have one string at 450ah and another at 330ah. I have a SW5548 what should I set the max charge rate at ? I have a generator, and solar PV.

[Sunking]

Robbie it depends on what technology you are referring too, and to some extent the manufacture/model.

Generically speaking maximum charge rate on flooded lead acid (FLA) batteries is 10 to 13% of the C20 amp hour rating. You will also see this expressed as C/10 to C/8 where C = the 20 amp hour rating. So for example a 100 AH rated battery max charge current is 10 to 13 amps. For FLA it is preferred to charge at the higher rates. Otherwise they can stratify and will need an alternate charge method periodically to stir up the acid and more frequent EQ charges. That can be a problem for off girders which requires a generator. At C/10 rate causes slight boiling of the electrolyte and needs less frequent EQ charges.

The sealed lead acid (SLA) is not so clearly defined as FLA. In the SLA family there are two basic types Gel and Absorbed Glass Mat. I wil not go into Gels as they are not well suited for Renewable Energy applications. In the AGM family it really depends on the manufacture. For example Trojan line can be charged as high as C/5 rate. While others can be charged as high as C/1, and at the other end of the spectrum no higher than C/20. So with respect to AGM, it depends on manufacture and model.

[russ]

Hi Robbie, Sunking has pointed out that there are few set rules for Gel and Absorbed Glass Mat batteries.

Try to search the manufacturers site on the net. They may have their instructions there for a particular battery series.

Russ

[Sunking]

Hi Robbie, Sunking has pointed out that there are few set rules for Gel and Absorbed Glass Mat batteries.

Try to search the manufacturers site on the net. They may have their instructions there for a particular battery series.

Russ

Thanks Russ for pointing that out. The real answer lies with your manufacture instructions.They should tell you the preferred charge rates and algorithm. Once you know that, then you can size the generator and AC charger required.

[greenHouse]

As Dereck pointed out (and be sure to follow russ's advice and consult the manufacturer's guidelines), there are some maximum rules.

In some applications, particularly when dealing with large amounts of autonomy ("no-sun days"), maintaining the maximum recommended rate of charge is impractical as it implies an overly large array and/or generator -- 780Ah is 78 amps @ ~60 volts (winter temp compensation voltage) or 5KW just for charging near the start of "Absorb" using a 5.5KW inverter (the generate has a 75A limit, so 78A is more than you've got).

Since you have two strings (of different capacities, which is a no-no), you have a fairly large total capacity which means you may be prone to stratification, particularly if you aren't reaching "Absorb" on a regular basis. In these situations, you want to make sure you can get into "Absorb" at a high rate of charge using both the generator and PV. Telling us the generator size would be helpful, as would the array size.

[robtribble]

response

The generator is 15kw@240, and they array is 3kw @100, and the minihydro is 1kw @ 48. There are 4 SW5548's and 3 MX60 solar controllers (2 for the array, 1 for the hydro)

so at 48 v I would have a max amps of :

62 @ 240 or ~ 120 amps @ 48
33 @ 100 or ~ 66 amps @ 48
20 @ 48 for a total of ~~ 200 amps @ 48 V

So I plan on splitting the generator charging amps across all 4 SW5548's

so what should the charge rate be, for how long, in EQ (maybe once per month), and how long in Bulk (once per week)


ps. calling me Robbie, sort of pisses me off ! I am not a kid. The name is Rob..I am 63, and been doing electrical stuff for 55 years

[greenHouse]

Rob,

I'd divide the available charging current equally among all four inverters, unless there is some reason not to get them all in on the fun. You will need to limit the total amps from the SW's since 200 amps total very likely exceeds the maximum permissible charging rate.

As for times, there are no hard and fast rules. The only way to accurately determine when to stop charging is experimentally. Since you have an MX-60, my suggestion would be to install a FLEXnet DC along with the rest of the OutBack kit and use it's AUX relay to provide signaling for when the batteries are fully, or mostly fully, and turn off the generator then.

I'll also offer up the products my company sells that can help you monitor what you've got. This is the product that I think, once you get that FLEXnet DC added, would give you the best understanding of what's going on. Although the instruction manual doesn't say so, the FLEXnet DC will accept 1000A / 100mv shunts. I'd go with two SW's per shunt, then all three MX-60's on the third. If you'd like to monitor all four SW's separately, we also support the APRS World Wind Data logger which supports shunt amplifiers, and it could monitor the SW's separately, but that gets a little weird ...

[Mike90250]

As for times, there are no hard and fast rules. The only way to accurately determine when to stop charging is experimentally. Since you have an MX-60, my suggestion would be to install a FLEXnet DC along with the rest of the OutBack kit and use it's AUX relay to provide signaling for when the batteries are fully, or mostly fully, and turn off the generator then......

I would modify this a bit, and it should save fuel. Let the genset run till the batteries drop out of BULK, about 80% full. At this point, the chargers drop out of high current mode, and switch to higher voltage. But your genset will now see only about 1/3 the earlier load. This will lead to a lot of fuel use, to do the last 20% of charging.
At 80%, you are out of the range where sulfation happens, and you can let solar top off the batteries, or wait till they get below 80% and run the genset in bulk mode.

Just trying to save fuel and engine hours.

[Jason]

hey rob, i'm moving this thread to the batteries forum

[Sunking]

The generator is 15kw@240, and they array is 3kw @100, and the minihydro is 1kw @ 48.

so what should the charge rate be, for how long, in EQ (maybe once per month), and how long in Bulk (once per week)

Rob what is the total capacity of the batteries? Is it 1 single string at 48 volts? What AH capacity? Sounds like your genset is more than adequate to do the job. I will give you exact numbers if you can provide the info needed.

• Battery operating voltage.
• Battery AH capacity.
• Number of strings

[Mike90250]

so what should the charge rate be, for how long, in EQ (maybe once per month), and how long in Bulk (once per week)

You have to get those #'s from the battery mfg, or rely on your charger to use generic settings.

Bulk: A preset voltage, about 14V, till current drops to some low value. Usually about 80% of capacity

Absorb A higher voltage for a preset time, which will bubble the electrolyte and de-stratify it. Mfg specs the time, 90 min - 3 hours range

EQ : Again, Mfg specs, higher voltage still, for 30 - 90 min, depending on mfg suggestions.

read ** All About Charge Controllers ** & ** All About MPPT Controllers ** for more info.

[greenHouse]

Mike,

Manufacturers can't specify the precise times he's looking for because they depend, among other things, on depth of discharge, health of the batteries, and most importantly -- bulk charging rate.

"Absorb" time from any given state-of-charge is a =constant= for a =constant= temperature. But the state-of-charge when the terminal voltage reaches the absorb voltage is determined by temperature, internal battery resistance and available current. Some of those numbers change from installation to installation and others just plain change over time.

[Sunking]

Mike,

Manufacturers can't specify the precise times he's looking for because they depend, among other things, on depth of discharge, health of the batteries, and most importantly -- bulk charging rate.

Which is C/10 to C/8 for FLA in most cases. C/10 is good enough for FREE INFORMATION on the web forums. Otherwise do your own work and look at something like the USER MANUAL. You get what you paid for.

[greenHouse]

Which is C/10 to C/8 for FLA in most cases. C/10 is good enough for FREE INFORMATION on the web forums. Otherwise do your own work and look at something like the USER MANUAL.

C/10 is just a number. C/8 is just a number. The minimum rate is "high enough to finish before I want to turn the d@mned generator off", so long as it's greater than about C/30. The maximum rate is, of course, what the manufacturer says. A lot of FLA's will take more than C/8 without harm, provided the temperature is watched like a hawk. The C/8 limit seems more a reflection of SLI batteries than true deep cycle.

I've not added the "charging rate curves" feature to my company's software, but eventually I'll be able to produce curves that show state-of-charge, charging rate, and time-to-completion. Because there aren't a lot of products that can answer this question without the user doing all the work themself.

[greenHouse]

Which is C/10 to C/8 for FLA in most cases. C/10 is good enough for FREE INFORMATION on the web forums. Otherwise do your own work and look at something like the USER MANUAL. You get what you paid for.

Okay, so you obviously wanted me to re-read this since you went back and added a bunch of highlighting and whatnot.

C/10 isn't a magic number. Let the guy tell us what he's got, then tell him if he'll be okay. For most systems, C/10 to C/20 is plenty fine. If he has a large bank and is running shallow cycles, even C/25 is plenty fine.

If you don't want to put in the effort, that's okay. I type north of 105WPM -- don't take me much time to put fingers to keyboard