Limitting battery charge rate

SolaRAC · May 2012

Is there a device that can sit between the battery and the charge controller that could be used to limit the charge rate?

Example:
Me panels are producting power over my ideal 13% charge rate. During the day, the additional power can perhaps be used with loads, but for obvious reasons, i don't want that extra power to go to my battery bank.

I'm sure those expensive mppt charge controllers can do something like this. Whats the poor man's solution?

Hope I'm not overthinking this...

westbranch · May 2012

Re: Limitting battery charge rate

what are the specs on your CC, panels and battery?

BB. · May 2012

Re: Limitting battery charge rate

The 13% is what we sort of recommend as a "cost effective" maximum for solar... You can go upwards of 25% rate of charge (done more often with a 120/240 VAC charger) as most folks are not discharging down to 20% state of charge and trying to recharge it back to 100% in ~8 hours (forklift type service).

If you do have very high rate of charge--Getting a controller that supports a Remote Battery Temperature Sensor will help prevent thermal run-a-way (battery gets hot, charging voltage falls, charger thinks battery needs more charging current, battery gets hotter--etc.).

-Bill

SolaRAC · May 2012

Re: Limitting battery charge rate

westbranch wrote: »

what are the specs on your CC, panels and battery?

I have a very small setup at the moment... Im just thinking ahead.

-75watt panel putting out 4.41 amps max.
-10 amp charge controller
-79ah battery.
-400 watt inverter

SolaRAC · May 2012

Re: Limitting battery charge rate

BB: What's the safest max charge rate for batteries? Oris this a battery specific question?

BB. · May 2012

Re: Limitting battery charge rate

Yes, it is a battery specific question... For "generic" Flooded Cell Batteries, about C/8 (~12.5%) rate of charge is "standard" and for forklift batteries, some brands offer internal cooling fans for higher rates of charge.

AGM batteries, some brands/model can accept upwards of C*4 rate of charge.

Again, the 13% is just a high/nominal value. You can control the charging by adjusting the Absorb charging voltage and using a Remote Battery Temperature Sensor can help prevent over charging/thermal run-a-way. Note that batteries do like to run cool for longer life--If your battery area is cool, they can probably take higher charging current vs being in metal building in direct desert sun.

Do you have a brand/model of battery you are thinking about? Any mfg. specifications?

And--I am not a battery expert--Just suggesting a starting point for your planning.

If you have consistent loading (say daily summer irrigation), you can assume the battery will always see Solar Current - Steady State loading. And, you can program some MPPT charge controllers with less than 100% of rated current (don't know any that will use a current shunt on the battery to limit charging current in real time).

Others here with more battery experience than I can probably give you some better answers...

-Bill

calbiker · May 2012

Re: Limitting battery charge rate

13% charge rate? That's way down in the mud. You can do way better than that.

The EV guys know how to charge batteries. They got the Ampere-Hour Law. Max charge rate = Battery capacity * (1 - SOC). If you got a 200AH battery discharged to 50% then max charge rate is 200AH * (1 - 0.5) = 100A. That's a 50% charge rate!

Personally, I don't care to charge my battery that fast. My converter (in boost mode) charges at 25%.

http://evbatterymonitoring.com/WebHelp/Battery_Book.htm#Section_3.htm

Cal

Photowhit · May 2012

Re: Limitting battery charge rate

SolaRAC wrote: »

I have a very small setup at the moment... Im just thinking ahead.

-75watt panel putting out 4.41 amps max.
-10 amp charge controller
-79ah battery.
-400 watt inverter

Is this your setup? one panel putting out 4.4 amps max into a 79 Ah (amphour) battery... or 5.56%

westbranch · May 2012

Re: Limitting battery charge rate

SolaRAC wrote: »

producing power over my ideal 13% charge rate.

Hope I'm not over thinking this...

Without looking at the make of equipment/battery you have, I do not see how you calculated you are exceeding 13%.
Please show us the calcs.

SolaRAC · May 2012

Re: Limitting battery charge rate

Yes. This is my first setup; just tryung to get an appreciation for how this stuff works. I dont depend on this setup in any way.

SolaRAC · May 2012

Re: Limitting battery charge rate

BB: Did you mean C/4 (not C*4) for AGM batteries?

If Im thinking correct, my 79ah AGM battery can safely charge at ~20amps?

Also, I'm considering purchasing two Trojan t-105 plus 6v 225ah batteries (to make 12 volts). What charge rate do you recommend here?

BB. · May 2012

Re: Limitting battery charge rate

No, I really did type C*4... From the NAWS Battery Faq:

[h=3]Battery Charging Voltages and Currents:[/h]
Most flooded batteries should be charged at no more than the "C/8" rate for any sustained period. "C/8" is the battery capacity at the 20-hour rate divided by 8. For a 220 AH battery, this would equal 26 Amps. Gelled cells should be charged at no more than the C/20 rate, or 5% of their amp-hour capacity. The Concorde AGM batteries are a special case - the can be charged at up the the Cx4 rate, or 400% of the capacity for the bulk charge cycle. However, since very few battery cables can take that much current, we don't recommend you try this at home. To avoid cable overheating, you should stick to C/4 or less.

Charging at 15.5 volts will give you a 100% charge on Lead-Acid batteries. Once the charging voltage reaches 2.583 volts per cell, charging should stop or be reduced to a trickle charge. Note that flooded batteries MUST bubble (gas) somewhat to insure a full charge, and to mix the electrolyte. Float voltage for Lead-Acid batteries should be about 2.15 to 2.23 volts per cell, or about 12.9-13.4 volts for a 12 volt battery. At higher temperatures (over 85 degrees F) this should be reduced to about 2.10 volts per cell.

Much of what I (we) try to do here is create a "balanced" system... Batteries that are not too big or too small, solar array, cabling, inverters, etc... If you do something way "out of balance", it can affect other parts of the system.

Not to say that somebody cannot do a C*4 rate of charge--But really want to ask why first--To ensure that the system, in total, makes sense.

-Bill

SolaRAC · May 2012

Re: Limitting battery charge rate

calbiker wrote: »

13% charge rate? That's way down in the mud. You can do way better than that.

The EV guys know how to charge batteries. They got the Ampere-Hour Law. Max charge rate = Battery capacity * (1 - SOC). If you got a 200AH battery discharged to 50% then max charge rate is 200AH * (1 - 0.5) = 100A. That's a 50% charge rate!

Personally, I don't care to charge my battery that fast. My converter (in boost mode) charges at 25%.

http://evbatterymonitoring.com/WebHelp/Battery_Book.htm#Section_3.htm

Cal

That's an interesting way of putting it... This sounds like an ever changing charge rate though. Need a special controller for that?

SolaRAC · May 2012

Re: Limitting battery charge rate

westbranch wrote: »

Without looking at the make of equipment/battery you have, I do not see how you calculated you are exceeding 13%.
Please show us the calcs.

I am not currently exceeding 13%; I was speaking hypothetically... Planning ahead.

For some reason I was undrr the impression that 13% was the safe ceiling

BB. · May 2012

Re: Limitting battery charge rate

C/8 is the, generic, safe ceiling for recharging Lead Acid Batteries. You can go upwards of C/4--But that will be less efficient (more heat into battery) as well as needing heavier cables, larger charging sources, you should have temperature monitoring, etc.

If you go above the generic rate, you need to do "more engineering" to ensure that everything plays together safely.

But, because, most of the time, nobody here will go below ~50% state of charge--the high current charge rates will not be for that long of time (before absorb voltage is hit and current starts to ramp down).

-Bill

niel · May 2012

Re: Limitting battery charge rate

calbiker wrote: »

13% charge rate? That's way down in the mud. You can do way better than that.

The EV guys know how to charge batteries. They got the Ampere-Hour Law. Max charge rate = Battery capacity * (1 - SOC). If you got a 200AH battery discharged to 50% then max charge rate is 200AH * (1 - 0.5) = 100A. That's a 50% charge rate!

Personally, I don't care to charge my battery that fast. My converter (in boost mode) charges at 25%.

http://evbatterymonitoring.com/WebHelp/Battery_Book.htm#Section_3.htm

Cal

the ev guys may know how to do it with the batteries they use. you can't arbitrarily apply that to all batteries as each manufacturer dictates what can or should be put into their batteries. even some agms have a smallish charge range like the deka line at i believe 30%. or was it 20% as i forget?

SolaRAC · May 2012

Re: Limitting battery charge rate

All very good to know.

I guess most of us dealing with solar will hit our pocket's ceiling before our charge rate ceiling.

If we were to all use the "Amp-hour Law" mentioned earlier (very interesting read by the way), we shouldn't have to worry about tempuratures.

Eric L · May 2012

Re: Limitting battery charge rate

I guess most of us dealing with solar will hit our pocket's ceiling before our charge rate ceiling.

Depends: as panel prices fall relative to battery prices, it gets easier to hit the charge rate ceiling. Whether it makes economic sense to do so is still an open question IMO, but I and perhaps some others have been experimenting with higher ratios of panels to batteries than usual.

calbiker · May 2012

Re: Limitting battery charge rate

niel wrote: »

the ev guys may know how to do it with the batteries they use. you can't arbitrarily apply that to all batteries as each manufacturer dictates what can or should be put into their batteries. even some agms have a smallish charge range like the deka line at i believe 30%. or was it 20% as i forget?

Yes, there are differences in batteries and their charging capabilities. But your guideline of 13% is way out of whack with those of the industry.

From Xantrex: "Generally, a wet cell battery should not be charged at a rate that exceeds 25% of its capacity."
http://www.xantrex.com/documents/Inverter-Chargers/Freedom-458/Freedom458%28445-0193-01-01_rev-2%29.pdf

There are other converter manufacturers that set the limit at 33% (C/3). One manufacturer provides a long list of batteries that can take that charge (Trojan being one of them).

Can you justify 13% max? I can understand Bill's remark that 13% may be the optimal upper limit for solar charging, but not for absolute maximum charge.

Cal

nsaspook · May 2012

Re: Limitting battery charge rate

calbiker wrote: »

Can you justify 13% max? I can understand Bill's remark that 13% may be the optimal upper limit for solar charging, but not for absolute maximum charge.

Cal

I'm sure you can charge at the higher rates but good engineering means you derate everything with large factor if your objective is the longest life of the system instead of absolute performance. With a EV having quick recharge times is a priority item with absolute battery life being lower on the list of design considerations. With a RE system design the trade-off normally would go to increased charge times vs higher cost of equipment and battery maintenance if at the end of the day the energy stored would be the same if the daily DOD is in the range of 20%.

SolaRevolution · May 2012

Re: Limitting battery charge rate

SolaRAC wrote: »

Me panels are producting power over my ideal 13% charge rate. During the day, the additional power can perhaps be used with loads, but for obvious reasons, i don't want that extra power to go to my battery bank.

I'm sure those expensive mppt charge controllers can do something like this. Whats the poor man's solution?

I suppose you could pull the fuse/ turn off the breaker from the "extra" strings of modules. That would be the "poorest" way to do it.

On the other side of town-

Too much PV is a common problem for large grid-tied battery systems. These systems are often designed to offset the grid power usage and the battery bank is often somewhat minimal. In these cases it can make good sense to dedicate a small portion of the array for a modest battery system and the have the rest of the array feeding the grid through non-battery inverters. Of course there are the end-of-the-worlders who can't stand the thought of having any unproductive PV should the SHTF. AC coupled systems with the ability to throttle down the PV output are attractive here, but the cost and hassels of doing a large split phase Sunny Island system can be a big disincentive.

In big DC coupled systems when there is more than one charge controller, I've found it useful to program them to different absorb set points.

One system I've worked with has 22kw of PV and a 1350 ah @ 48v battery (near the (C3) range). When the grid goes down the battery voltage spikes hard and fast. The solution I found that seems to work okay is to set 3 "tiers" in the voltage setpoints of the 8 Outback MX60 charge controllers. With an inverter Sell voltage of 55vdc the first three MX60s are set at an absorb of 56.4v. The next 3 are set at 57v and the last 2 are set at 58v.

Alex Aragon

calbiker · May 2012

Re: Limitting battery charge rate

nsaspook wrote: »

I'm sure you can charge at the higher rates but good engineering means you derate everything with large factor if your objective is the longest life of the system instead of absolute performance.

Absolutely, it's always good to derate. Good engineering practice also means that there's a documented analysis showing just how 13% was established.

BTW, why does the RV industry install 45A or 60A converters along with 220AH batteries? That's way over 13%.

Cal

nsaspook · May 2012

Re: Limitting battery charge rate

calbiker wrote: »

BTW, why does the RV industry install 45A or 60A converters along with 220AH batteries? That's way over 13%.

Cal

Because the RV industry like the EV industry has fast recharge as a top priority and the usage pattern is similar (run the battery down to nothing then recharge) so then all of the 45 amps in bulk charge is converted to stored energy at a 90+% overall efficiency and not the 50% efficiency when recharging from 80% to 100% on a typical RE system. High current recharges are mainly only useful when the (FLA) battery is almost flat (less than 60%) and will convert the electrical energy to chemical without causing resistive heat stress from the "electronic" part of the internal resistance and while the ionic internal resistance of the plates and electrolyte is low.

Cariboocoot · May 2012

Re: Limitting battery charge rate

The 13% upper limit is based somewhat on economics, as more panels equates to more money. When panels were really expensive this was a major problem. Not so much so anymore.

For 220 Amp hours of 12 Volt at 13% rate you'd need about 445 Watts of panel. At today's prices that's like $715. At 10% you'd have 343 Watts of panel for about $550. Difference of $165.

At the "old" prices it was more like $2,000 vs. $1,000.

niel · May 2012

Re: Limitting battery charge rate

calbiker wrote: »

Yes, there are differences in batteries and their charging capabilities. But your guideline of 13% is way out of whack with those of the industry.

From Xantrex: "Generally, a wet cell battery should not be charged at a rate that exceeds 25% of its capacity."
http://www.xantrex.com/documents/Inverter-Chargers/Freedom-458/Freedom458%28445-0193-01-01_rev-2%29.pdf

There are other converter manufacturers that set the limit at 33% (C/3). One manufacturer provides a long list of batteries that can take that charge (Trojan being one of them).

Can you justify 13% max? I can understand Bill's remark that 13% may be the optimal upper limit for solar charging, but not for absolute maximum charge.

Cal

you are correct that for many batteries this may be too conservative, but i can't tell everybody to charge at a high rate when not all batteries can be at a higher rate. i always say that the manufacturer's recommendations should be gotten for a top end charge. even if they can take a higher charge rate one must understand that it could mean much higher maintenance and possibly a shorter lifespan in so doing.

btw, xantrex is not the ones setting the battery industry's standards.

vtmaps · May 2012

Re: Limitting battery charge rate

Is it fair to assume that manufacturer's recommendations for max charge rate are based on batteries being at standard temperature? If so, a rule of thumb max recharge rate (such as 13%) would have to be conservative to allow that batteries might be warmer than standard temperature.
--vtMaps

niel · May 2012

Re: Limitting battery charge rate

charge rates to batteries at other temps should be the same. the current required doesn't change at other temps as it is the voltage that gets an adjustment under different temps.

vtmaps · May 2012

Re: Limitting battery charge rate

niel wrote: »

charge rates to batteries at other temps should be the same. the current required doesn't change at other temps as it is the voltage that gets an adjustment under different temps.

My point was that high charge rates make heat. If a battery is already hot, the max charge rate must be reduced. That is why a rule of thumb (13% max charge rate) needs to be conservative. Maybe the batteries can take more than 13% when they're cool, but for a rule of thumb you must consider that they might be hot.
--vtMaps

Cariboocoot · May 2012

Re: Limitting battery charge rate

vtmaps wrote: »

My point was that high charge rates make heat. If a battery is already hot, the max charge rate must be reduced. That is why a rule of thumb (13% max charge rate) needs to be conservative. Maybe the batteries can take more than 13% when they're cool, but for a rule of thumb you must consider that they might be hot.
--vtMaps

You are correct that current makes heat, which is why high charge rates damage batteries over time.
But the temperature affects the Voltage not the current, as Niel said. An RTS on a charger will increase Voltage to cold batteries, reduce it for hot batteries. Leave that out and you could be in trouble from too high Voltage bubbling electrolyte away.

Every spec for every piece of electronic equipment comes with a temperature tag, like panels' maximum output or batteries' Amp hour capacity.

And let's not forget that higher current requires bigger wires and fuses and ultimately is more difficult to handle. Hence the repeated advice about increasing system Voltage rather than current to achieve greater power.

Batteries (of the type typically used in RE) actually prefer the charge profile they get from solar; starting at low current when the panels haven't enough sun for full output, then both Voltage and current rising as the battery charges and the panels' output increases, then tapering the current off to maintain Absorb Voltage, then easing off to a Float Voltage for as long as it can be maintained.

Dumping large amounts of current into FLA's or even AGM's is a sure-fire (Hah!) way to cut years off their lives. Not recommended. Do it at your own risk, and be sure you have good insurance.

calbiker · May 2012

Re: Limitting battery charge rate

Cariboocoot wrote: »

Dumping large amounts of current into FLA's or even AGM's is a sure-fire (Hah!) way to cut years off their lives. Not recommended. Do it at your own risk, and be sure you have good insurance.

You're doing a disservice using scare tactics without providing hard facts. RV'ers typically get 6 to 8 years use out of GC batteries. And they are not charging at C/8, not even close.

Your battery charging FAQ doesn't have much credibility when stating "Most flooded batteries should be charged at no more than the "C/8" rate for any sustained period." In bulk mode, most flooded batteries can be charged at C/4 or 25% of capacity.

The way the FAQ is written now may cause people to scrub current just to keep battery charge under 13%. That would be absurd!

Cal

Cariboocoot · May 2012

Re: Limitting battery charge rate

Okay Cal; you try it. Let us know how it works for you.

I and others here have pointed out all the reasons for the 5-13 % rule-of-thumb including that the battery manufacturers have their own recommendations which may fall outside those parameters. For instance it is well-known that AGM's can take more current than FLA's.

I will continue to recommend what I know works and is both practical and safe. The hard facts I base my position on is decades of my own experience, including pushing batteries beyond their limits to see how they fare. Saying that this is "absurd" and "scare tactics" is neither true nor appreciated. If you had made the same remarks about anyone else you would be suspended.

Limitting battery charge rate

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