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How Fast Can You Charge AGM Batteries?

4K views 12 replies 5 participants last post by  Maine Sail 
#1 · (Edited)
Hi All,

Finally got around to doing some testing & data logging that I have been meaning to do for a long, long while. The article hits those proverbial questions that are almost always assumed or answered incorrectly such as:

How fast can I charge an AGM battery?

What SOC can I charge to in 1 hour or 2 hours?

At what % SOC do I end bulk and hit absorption voltage?

Does a large charge current really make a difference?

How long does it take to charge from 50% SOC to 100% SOC?


How Fast Can an AGM Battery be Charged (LINK)

Hope this helps answer a few of those nagging questions....

P.S. Still editing out some typos etc.. If you find those buggers let me know!

.
 
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#2 ·
Nice study, Maine. The bottom line, as I read it, is that going with a bigger alternator has nonlinear benefits if you routinely drop to 50% SOC. An alternator that just barely achieves 0.2C will be running flat out for long periods of time and will probably have a short life. On the other hand, one that gets to 0.4C won't be breaking much of a sweat after 20 minutes. Given how the price of alternators scales rapidly with charge capacity (including special belting, mounting, etc), maybe the most economical solution is something in between.

I've got a different scenario than the test cases you studied and it may be common enough to consider testing it as well.

When I bought my boat it already had a big ~800Ah Lifeline AGM bank (4 x 4D). It also had a Delco 140A alternator driven with twin 3/8" V belts and controlled by a Balmar regulator. That means I'm not even achieving 0.2C. However, because the bank is so big and because I've got 400W of solar, I seldom get much below 80% SOC (70% at the lowest) before I'm running the engine to get somewhere in light air and charging for an hour or two along the way.

I'd be curious to see how your test bed behaves at 0.15C for an hour or two starting at a DOD of 80%. My boat may not be typical as far as bank size and charge capacities go but a boat with a 450Ah house bank, an 80A alternator, and 225W of solar would have similar relative specs so this scenario might be of interest to many.

Thanks again for all the work you do for the community!
 
#13 · (Edited)
Nice study, Maine. The bottom line, as I read it, is that going with a bigger alternator has nonlinear benefits if you routinely drop to 50% SOC. An alternator that just barely achieves 0.2C will be running flat out for long periods of time and will probably have a short life.
Yes the smaller the alternator the longer it's in bulk and thus the hotter it gets. Too much heat eventually kills alternators. I have physically measured alternators in excess of 300F and about 270F is the absolute max, with about 230F-240F being the optimal maximum.

On the other hand, one that gets to 0.4C won't be breaking much of a sweat after 20 minutes. Given how the price of alternators scales rapidly with charge capacity (including special belting, mounting, etc), maybe the most economical solution is something in between.
Fair assessment and also consider that .4C on a typical 450Ah bank is a charge rate of 180A... .4C on your bank is 320A. I believe their is a sweeter spot between .25C & .35C that balances cost, charge speed and reality. Charge times to 100% don't change much with a higher charge rate but they do in short duration bursts, eg; 1 hour, like we do on sailboats.

I've got a different scenario than the test cases you studied and it may be common enough to consider testing it as well.

When I bought my boat it already had a big ~800Ah Lifeline AGM bank (4 x 4D). It also had a Delco 140A alternator driven with twin 3/8" V belts and controlled by a Balmar regulator. That means I'm not even achieving 0.2C. However, because the bank is so big and because I've got 400W of solar, I seldom get much below 80% SOC (70% at the lowest) before I'm running the engine to get somewhere in light air and charging for an hour or two along the way.

I'd be curious to see how your test bed behaves at 0.15C for an hour or two starting at a DOD of 80%. My boat may not be typical as far as bank size and charge capacities go but a boat with a 450Ah house bank, an 80A alternator, and 225W of solar would have similar relative specs so this scenario might be of interest to many.
The .15C rate can be scaled pretty accurately. A charge rate of .15C on a 100Ah battery is 15A but on an 800Ah bank it is 120A. Both charge rates are still 15% of the Ah capacity.

Your alt will be in bulk for a very short duration starting at 80% SOC and your charging times will have smaller one hour or two hour gains, in returned energy, due to charge efficiency not being linear and being considerably worse the higher you go in the SOC curve. Nothing wrong with cycling high in the SOC curve but your net returns on usable energy will simply require longer charge times. When your engine or gen charging, in net accepted current, approaches what your PV array can supply in current, stop the engine or gen and let the solar take over...
 
#3 · (Edited)
Maine Sail:

Interesting study I'm trying to figure out where the charging system for my 48 volt electric propulsion bank fits your research.

My Zivan NG-1 48 volt charger outputs around 15.5 Amps max to charge a 48 volt battery bank consisting of four 8A4D batterries in series. What would you calculate be the charge rate with this setup?

I'm thinking about buying another charger that can output 25 amps which should decrease charge time according to your research.

Since I usually charge the bank using the Honda 2000i generator I am limited in the amount of current a charger can provide so as to not trip the generators breaker during bulk charging.

I use the NG-1 for the bulk charge and switch to a Dual Pro PS4 charger when the bank reaches 94% SOC. I know when to switch because the NG-1 pulses the battery bank and the Honda starts to rev in relation to the charger current pulses. The PS4 charger charges each battery in the bank individually and seems to keep the battery bank in balance. This method has been working great for eight years.

One note I almost never have bought my battery bank down to 50% SOC. Usually I switch to hybrid mode when I see the battery SOC is around 75% and start charging the battery bank as soon as the anchor is dropped if not before to 100% SOC. So far the batteries have been holding up well using this system.
 
#4 ·
My Zivan NG-1 48 volt charger outputs around 15.5 Amps max to charge a 48 volt battery bank consisting of four 8A4D batterries in series. What would you calculate be the charge rate with this setup?
You have a 210Ah 48V bank so 15.5A is a .074C charge rate...
 
#7 · (Edited)
Here's a graphic example of one of MaineSail's points, i.e., what happens to charge acceptance when you use different sized battery chargers?

Data were derived from tests I did a few years ago with Concorde/Lifeline battery company, using a new GPL4D AGM battery.

Four different-sized chargers were used after battery had been fully charged, then drained to 50%SOC. Data were recorded each minute for the first two hours.

The area under each colored line approximates the quantity of energy put back into the battery.

You can see that the 200% and the 50% sized chargers (blue and red, respectively) reached the same level of charge acceptance (amperage) after about 90 minutes, whereas the 25% and the 10% sized chargers (yellow and blue, respectively) took about 115 minutes to reach the same levels of charge acceptance (amperage), though this level was considerably higher than the other two larger chargers. None of the four managed to reach full charging levels in this 2-hour period.

Bottom line: larger charging capacity is a good thing for replacing energy fast in the early stages of charging. BTW, Concorde's own research says this is better for the battery, too.

However, reaching a truly full charge still takes several hours.

Text Line Font Slope Parallel


Bill
 
#9 ·
BTW, Concorde's own research says this is better for the battery, too.
Dave V. Lifelines lead engineer actually authored a study on this years ago. Bottom line is that higher charging current leads to longer battery cycle life. Neither Odyssey/EnerSys or Lifeline is willing to quantify by how much though..

However, reaching a truly full charge still takes several hours.

Bill
The reason I did my testing is because the AGM makers have not openly published this and certainly not with a slightly used battery that represents a more real world data point.. That 2 hour testing is a sweet spot that shows good performance. I could get to 96% SOC in 2 hours at .4C but it took another 3.5 hours to get to 100% SOC. When you consider this it is no wonder they don't publish the last few % and how long that takes..

In the charge rates we typically have on boats 20% to 40% +/- I found very little difference in the total time to 100% SOC between 50% and 100%.
 
#10 ·
A couple of other observations that are interesting on stored energy at .4C.

-From 50% SOC in one hour charged at .4C we could store 33.43 Ah's and attain approx 85% SOC

-From 50% SOC in two hours charged at .4C we could store 44.48 Ah's and attain approx 96% SOC

This means the second hour of charging at .4C only netted a gain of 11.05 Ah's and shows how much the charge efficiency drops off as SOC increases. The second hour of engine or generator run time at .4C only returned 11% of capacity yet the first hour returns approx 35% of capacity....
 
#12 ·
It depends :)

Mostly on....

. state of charge (SOC); and

. health of battery

In general, AGMs will accept a lot more recharging current than will flooded lead-acid batteries (FLA) in a given time period, especially when deeply discharged.

FLA's generally will accept only 20-25% of their rated AH when discharged to 50%. That is, a 100AH FLA will accept only 20-25A charge current to begin with.

AGMs can accept a lot more. Discharged to 50% SOC, they can accept more than 100% of their rated AH initially (see graph above). That's four or five times as much as FLAs at the same SOC.

Discharged to 20% SOC, AGMs can accept more than 5 times their rated AH initially. That's 500A for a 100AH AGM discharged to 20% SOC. Almost no boat has the ability to provide this much charging current. Most have only enough to provide 10% to 30% or so of rated AH.

Both AGMs and FLAs require several hours of charging to reach 100% SOC. AGMs get there faster, but it still takes hours....more than is practical for most boats to charge by mechanical means. That's why solar panels, wind generators, etc. are a good investment.

Bill
 
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