Limitting battery charge rate

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Comments

  • nsaspooknsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Limitting battery charge rate
    calbiker wrote: »
    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.

    Apples and Oranges. Not many RVers are on the road 365 days a year and cycle batteries every day during that period. Usage lifetimes vary but it's closely linked to D/C cycles and DOD. If you use your RV and cycle the battery to 80% DOD every two days for a few months during the summer they just might last 5 years and have 600 cycles. In a RE system the same 600 cycles at 30% DOD could happen in only 2 years with the battery still having several hundred more cycles left of usable life.

    cyclelife2.gif
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: Limitting battery charge rate
    nsaspook wrote: »
    Apples and Oranges... Usage lifetimes vary but it's closely linked to D/C cycles and DOD. If you ... cycle the battery to 80% DOD every two days for a few months during the summer they just might last 5 years and have 600 cycles. In a RE system the same 600 cycles at 30% DOD could happen in only 2 years with the battery still having several hundred more cycles left of usable life.

    Apples and oranges for sure!

    What is the right way really depends on what you trying to accomplish. Battery longevity based on number of cycles is not the only goal in system design. In some cases the "ideal" charge recommendations will not have a positive effect on the economics or usefulness of a system design. There are so many different factors at play that maybe there should be different "rules of thumb" for different situations. After all, you have a thumb "on the other hand" too.:blush:

    What is right, and recommended for a solar powered off-grid full-time residence is different for a UPS system, a weekend cabin, an EV charger, a system with a 24/365 hydro turbine, etc.

    -A full time off gridder will be looking at their system's PV capacity for daily charging. Their biggest concern may be how often they need to replace their batteries.

    -If you have a back-up system for emergency use (hurricanes, earthquakes...) you will want to get the best use of your generator run time because fuel can be scarce.

    -If you have a weekend cabin with enough PV for a finish charge, again you may want to max out your charge rate to decrease generator run time. The total number of cycles would be less important because even if you use the system every weekend it may only be 100+/- cycles/year.

    -A ups or grid-backup system may hardly ever cycle, but the few times it does, you want it to recharge quickly so it is ready for the next utility outtage as quickly as possible.


    Sure, I know, the "battery charging rules of thumb" are intended to be a guideline for battery longevity based on the manufacturer's recommendations. Just keep in mind that it may not be best to hang onto this as the right approach for every situation.

    Alex Aragon
  • nsaspooknsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Limitting battery charge rate
    Apples and oranges for sure!

    What is the right way really depends on what you trying to accomplish. Battery longevity based on number of cycles is not the only goal in system design. In some cases the "ideal" charge recommendations will not have a positive effect on the economics or usefulness of a system design. There are so many different factors at play that maybe there should be different "rules of thumb" for different situations. After all, you have a thumb "on the other hand" too.:blush:

    Most of us do understand that but this is a RE forum and most of the readers would be looking for information about batteries performance with that in mind so the design recommendations for moderate charging rates seem appropriate as a starting point. We have forklift battery charge systems with max recharge rates that would melt a normal battery but that same battery used in a RE system would almost never see or need that because the peak energy requirement to fast charge that system would cost a mint and be underutilized most of the time.
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: Limitting battery charge rate
    SolaRAC wrote: »
    Is there a device that can sit between the battery and the charge controller that could be used to limit the charge rate?

    You need two things - a means to sense current and a means to reduce current.

    The sense current part is easy. There are a large number of in-line (resistive) and thread-through (Hall effect) current sensors that can be used to measure current going to the battery.

    The control part is a little harder. On a charge controller with remote sense terminals you could artificially increase the voltage at the sense terminals. This would make the charge controller think that the battery was fully charged, and it would thus reduce current to prevent overcharging. A less-good way to do it would be to mess with the temperature sensor. If you increase the apparent temperature sensed (by decreasing the resistance connected to the thermistor input) you will cause the charge controller to think the battery is hotter, and thus it will back off on charge _voltage._ This will help near full charge, but if the battery is low, even a very high apparent temperature might not be sufficient to back off voltage enough - and you also run the risk of tripping the charge controller's overtemperature limits.

    This is an interesting question because battery charge capacity is what limits how large a panel you can use with a small battery bank. Someone who has a 10kW grid tie system who wants just a few kwhr's of backup currently has a problem - because if the grid goes down and all that power is dumped to the battery they will see 2-5C charge rates, and most batteries can't go over C/3. If you had a way to reduce max charge current to the BATTERY (not the system, just the battery) that entire 10kW would still be available to the inverter, and the system will limit the power only when there is little to no load.
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    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.
    I think you're still missing my point. My point has nothing to do with voltage or temperature compensation. It is only about max charge rate. Is my writing so unclear?

    Many chargers (either solar charge controllers or inverter-chargers) can be programmed to limit the charging rate. As a general rule of thumb it is probably a good idea to limit that rate to 13% of the batteries AH capacity. One reason to use that 13% number is that under most circumstances and most battery types it is a safe number to use. If I were to devise a rule of thumb for unheated battery sheds in Canada in the winter, I would probably suggest a higher than 13% safe charge rate because I am not so concerned about overheating the batteries.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: Limitting battery charge rate
    vtmaps wrote: »
    If I were to devise a rule of thumb for unheated battery sheds in Canada in the winter, I would probably suggest a higher than 13% safe charge rate because I am not so concerned about overheating the batteries.

    Hmm.

    Is charge induced heating always bad? Or in the case of cold batteries in Nordic winter power rooms is the heat produced by high charge rates beneficial as long as you don’t exceed 25ºC / 75ºF? (If so, I'm sure there are still maximum current limitations.)

    On the other hand, it's my understanding that a hot battery can accept higher charge and discharge current but it does more "wear-and-tear" damage. By adjusting the charge setpoint voltage in charging equipment, the battery bank will still be able to accept a higher charge rate (more wear-and -tear) but it will end at a lower voltage.

    Would it maks sense to also have charge rates temp compensated?

    Alex Aragon
  • calbikercalbiker Banned Posts: 50 ✭✭✭✭
    Re: Limitting battery charge rate
    vtmaps wrote: »
    I think you're still missing my point. My point has nothing to do with voltage or temperature compensation. It is only about max charge rate. Is my writing so unclear?

    If I were to devise a rule of thumb for unheated battery sheds in Canada in the winter, I would probably suggest a higher than 13% safe charge rate because I am not so concerned about overheating the batteries.
    --vtMaps

    I charge my GC batteries at 25% all the time. There is no noticeable increase in battery temperature.

    My two 6V batteries have about 15 m ohm resistance during charge. They are charged with 55A. Power consumed is:

    p = I^2 * R
    P = 55^2 * 0.015
    P = 45 W

    Each battery is heated by 23 W, or 7.5 W per cell.

    If you think heating is such a huge issue at C/4, then provide some evidence.

    Cal
  • BB.BB. Super Moderators, Administrators Posts: 32,088 admin
    Re: Limitting battery charge rate

    GC is gell cell? [see from Niel, you may mean Golf Cart]

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Limitting battery charge rate

    could be golf cart. at least these will take a higher charge better than gels.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Limitting battery charge rate
    BB. wrote: »
    GCis gell cell?

    -Bill

    Golf Cart more likely, Bill.

    They'll take it. Lots of batteries will. Some won't. And as has been mentioned before the application has a lot to do with how it's recharged.
    Since, as has been pointed out, we deal with renewable energy systems on this forum other applications really don't enter into the discussion. Car batteries get discharged heavily perhaps several times a day, but not deeply and they are (we hope) recharged very quickly immediately after. They last for years in such use. Different type of battery, different application. Not applicable to RE systems.

    As for purposefully using high charge currents to warm batteries in cold climates, it isn't necessary. A charged battery won't freeze, and unlike car batteries in the same climate RE batteries don't sit getting cold and then have a heavy load demand placed on them (unless the system is totally wrong). Ambient temperature does affect the rate at which the battery will radiate its internal temperature. But simply insulating the batteries against thermal loss/gain (thereby achieving a more consistent battery temp year-round). Normal charge rates are quite sufficient to keep battery temp at active levels.

    I will continue to recommend what I know works and is cost-effective and safe. Anyone is free to disagree with my POV and implement their own charging regime. But the next person who gets "snarky" gets a week off. Be civil.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Limitting battery charge rate
    But the next person who gets "snarky" gets a week off. Be civil.

    snarky snarky

    can i have a week off?;):p oops, i said it twice so does that mean 2 weeks?:-):p
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Limitting battery charge rate
    niel wrote: »
    snarky snarky

    can i have a week off?;):p oops, i said it twice so does that mean 2 weeks?:-):p

    You know the punishment for moderators: more work! :cry:
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Limitting battery charge rate
    You know the punishment for moderators: more work! :cry:

    oh well, what can i say as i tried. :cry:
  • BB.BB. Super Moderators, Administrators Posts: 32,088 admin
    Re: Limitting battery charge rate

    Not trying to tell people that their batteries WILL self destruct at C/4 charge rates--Just recommending understanding and even, possibly, doing tests to ensure that all is safe.

    For an off grid system... A C/4 charge rate and people discharging by 25% per day (2 days of autonomy and 50% maximum discharge)--Means that you are looking at attempting to recharge from solar inside of 1 hour--Certainly possible to attempt (but lead acid batteries will tend to self regulate to below C/4 rate of charge once the battery bank is at 80% to 90% state of charge with "typical" recharging voltage settings).

    If you have heavy loads during the day (pumping/shop/etc.) and light/no loads at night--Then a smaller bank and larger array may certainly make sense. (obviously, special cases were you have limited sun may also make good sense).

    But if most of your loads are at night and charging during the day, then a smaller array will probably have a higher "usage factor" (more Watt*Hours used per Watt rating)--and will still recharge your bank.

    The other thing I try to do is recommend an AC charger/AC generator such that you can keep the generator running at 50% or higher load rating for good fuel efficiency (most generators run about 50% fuel flow between 50% to near 0% rated loads).

    If you have a high rate of charge and a "typical" off grid usage pattern--It is unlikely you would keep C/4 charge rates for long periods of time (one rule of thumb is that "bulk" time would equal "absorb" time to roughly 2-4 hours on absorb).

    Here is a good thread on "matching" a generator charging solution to the battery bank (this is a smaller setup with a Honda eu2000i/1,600 watt generator):

    Question about battery charger selection with EU2000 generator.


    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • calbikercalbiker Banned Posts: 50 ✭✭✭✭
    Re: Limitting battery charge rate
    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.

    You are twisting my words! You are using scare tactics when arguing the possibility of fire and needing good insurance. AGM thermal runaway is extremely rare. Furthermore, I said it would be absurd for someone to throttle back charging current because they are 5% over your mystical 13% marker.

    I spent 30 years designing electronics for battery powered vehicles (cars, tanks, airplanes and manned space travel). I understand batteries.

    I'm still waiting for hard facts that charging a deep cycle battery at 20% is detrimental.

    Cal
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: Limitting battery charge rate
    calbiker wrote: »
    I'm still waiting for hard facts that charging a deep cycle battery at 20% is detrimental.

    Calbiker, this discussion is one where everyone seems to be missing the points. I agree that a 20% charge rate is usually not detrimental. I also feel that a general rule of thumb must specify a max charge rate that is always safe for all batteries under all conditions. Therefor, I agree that a 13% max rate is a good rule of thumb.

    Several forum members (including myself) have agreed that 13% is not an absolute rule for all situations. But it is a good rule of thumb! That's all it is.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • calbikercalbiker Banned Posts: 50 ✭✭✭✭
    Re: Limitting battery charge rate
    vtmaps wrote: »
    I also feel that a general rule of thumb must specify a max charge rate that is always safe for all batteries under all conditions. Therefor, I agree that a 13% max rate is a good rule of thumb.
    --vtMaps

    I wouldn't consider that a good rule of thumb. I would not apply the characteristic of a poorly engineered battery to limit all other batteries capabilities. Just think how silly that would be if you used Harbor Freight panels for the basis of solar panel rules of thumb.

    Cal
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Limitting battery charge rate

    I didn't twist your words; I quoted you verbatim.

    Never did I say anything about thermal runaway of the batteries starting fires. There are other considerations with current, mainly the wiring and connections.

    The 5-13% rule-of-thumb is a good and practical one in that it will work reliably with most any battery. We repeatedly tell people to check manufacturer's recommendations for charge rates and Voltages. Nothing "mystical" about it at all.

    Some RE systems have 1000 Amp hour battery banks, in which case 20% charge rate would be 200 Amps. Wiring and circuit protection gets larger the higher the current. People have problems with keeping connections tight even at lower current.

    No one has ever said a 20% charge rate or even higher is definitely detrimental to a battery. It depends on the battery and the application.

    We have to deal in generalities here. What most people are after is getting a system that works. They are not interested in pushing things to absolute limits, nor is it wise to do so. In the practical application, the 10% rate works even within the short 4 hour window of sun. Sometimes the 5% rate is sufficient for a system. Why buy more panels/controller capacity/wire size et cetera than you actually need? Most of the time we have difficulty convincing people to increase their panels enough to get the 10% rate. I can't imagine trying to get them to buy twice as much, even at today's low prices. Actually I can, because the higher charge rates are really good for the tall case L16 batteries.

    If you can't accept this, that is too bad.
  • calbikercalbiker Banned Posts: 50 ✭✭✭✭
    Re: Limitting battery charge rate
    ... What most people are after is getting a system that works. They are not interested in pushing things to absolute limits, nor is it wise to do so. In the practical application, the 10% rate works even within the short 4 hour window of sun. Sometimes the 5% rate is sufficient for a system. Why buy more panels/controller capacity/wire size et cetera than you actually need? Most of the time we have difficulty convincing people to increase their panels enough to get the 10% rate. I can't imagine trying to get them to buy twice as much, even at today's low prices. Actually I can, because the higher charge rates are really good for the tall case L16 batteries.

    I fully agree. I wouldn't recommend going over 10% on a solar install. However, that is not the issue here. As others pointed out, you are doing a apples to oranges comparison. The maximum current a battery can safely handle is totally different than the maximum current a solar array should charge a battery. Using generator and Iota converter, I charge my golf cart (GC) batteries with 55A, while solar charges at 5A.

    Cal
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Limitting battery charge rate

    We often recommend that around here. After all, what is the sense of firing up a gen if you don't load it up as well? Burn the gas for a quick Bulk (and whatever percent of Absorb you can do before the current falls off and the gen stops being economical).

    The exception to this is cheapskates like me who use small inverter-generators and limit the output because I won't stick a crowbar in my wallet and buy a bigger generator. :p
  • calbikercalbiker Banned Posts: 50 ✭✭✭✭
    Re: Limitting battery charge rate
    We often recommend that around here. After all, what is the sense of firing up a gen if you don't load it up as well? Burn the gas for a quick Bulk (and whatever percent of Absorb you can do before the current falls off and the gen stops being economical).

    If you also recommend higher charging rates when using a generator then this should be reflected in your FAQ. Right now it says max current is only C/8. There is a moderator here that's adamant (on another forum) that C/8 is the absolute limit for generator driven charging. He quotes this FAQ as being the bible!

    In general, most batteries can be safely charged at C/4.

    Cal
  • VicVic Solar Expert Posts: 3,117 ✭✭✭✭
    Re: Limitting battery charge rate

    All things about battery care and charging are relative, and there are hundreds of exceptions.

    First follow the manufacturer's recommendations for the exact battery in question.

    Personally I'd never charge MY batteries at 25% of 20-hour Capacity. That is just me. Very, Very high charge rates seem very very risky IMHO. FWIW Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Limitting battery charge rate

    I have just checked the battery FAQ's. It says: "Most flooded batteries should be charged at no more than the "C/8" rate for any sustained period. " There is no mention of C/8 being the maximum. Since the FAQ's information is accurate it will not be changed.

    I don't know what moderator here is adamant about C/8 being the max on this or any other forum. There is nothing in the FAQ stating that C/8 is the limit for generator charging either. This is a warning about exceeding the maximum capacity on gel cells and that it is easy to exceed the manufacturer's limit when using a generator.

    The 5% to 13% range is recommended (and will continue to be on this forum) because it is sufficient for recharging most batteries used in RE without exceeding manufacturer's specs and without spending more than is necessary on panels & controller.

    Trojan batteries states: "A good charging rate for a battery with a 20-hr capacity of 225 amp-hr is about 22 (10%) to 29 (13%) amps. You can of course go slightly higher or lower depending on what is available on the market."

    I trust their word on the matter.
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