How do I know amp hours going to battery?

System
System Posts: 2,511 admin
I have a new solar setup. 55w solar panel, charge controller, inverter, all set up.

I have measured voltage on panel in the 18volt+ range --- all ok.

I simply don't know how to use the multimeter to determine how much I am charging the battery. Of course, I can measure the battery at the end of the day and compare readings before charging, but can I see how much I'm actually charging in 'real time' and know how many amp-hours I'm replacing in the battery?

I realize the sun will come and go, angle changes, etc. Just want to know, at a given point, how much charging I'm getting and to change angle of panel.

Thanks. I know this will be simple.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How do I know amp hours going to battery?

    Assuming your solar charger does not have a current meter:
    • Battery Monitor--with a "shunt" in the negative battery lead, designed to keep track of Amp*Hours of current flow in/out of battery bank to give an estimate of the current State of Charge of the battery bank. Bi-directional measurements (add/subtract from master total).
    • Amp*Hour / Watt*Hour meter--frequently used by RC (radio control) and other hobbyists to measure voltage, current, current*time, and watts*time of flow to or from a battery (one way only for these models.
    • Current Shunt and millivolt meter... Place a precision resistor in battery negative leg and use a volt meter to display current flow (shunt is exactly the same part used with a Battery Monitor).
    • If the current flow is less than 10 amps maximum, you can put a DMM in the negative lead of the battery. Note--if you do this wrong, it is very easy to destroy your DMM or worse.
    And to address another unasked question--To measure AC power / kWH--a Kill-a-Watt meter is a great piece of equipment to use around the home (find those energy hogs).

    And for measuring whole home AC power usage--there are various units, like the T.E.D. for doing that. Very nice if you have large stuff (electric stove, well pump, A/C, etc.).

    The above are just starting points for you to take a look at. There are many options out there and some may be better for your needs than others.

    Please feel free to ask more questions.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: How do I know amp hours going to battery?

    Just an observation:

    55 Watt solar panel is not very big. You could expect to harvest about 60 Watt hours per day. It would be adequate for charging a battery no larger than 60 Amp/hrs (12 Volt).

    Good place to start learning about solar. :D
  • Highland_Fling
    Highland_Fling Solar Expert Posts: 37
    Re: How do I know amp hours going to battery?
    Just an observation:

    55 Watt solar panel is not very big. You could expect to harvest about 60 Watt hours per day. It would be adequate for charging a battery no larger than 60 Amp/hrs (12 Volt).

    Good place to start learning about solar. :D

    Well i thought i was learning but i cant get your figures to make any sense - do help me please

    55W x 9 hours = 495 W/H x .77 for real life figures = 381 W/H assuming 15V to charge the battery 25 A/H

    OR

    55W x .77= 42.35W /15 =2,8A times the number of hours you want to suggest the panels will see full sun


    SURELY this is all about A/H out and A/H in and while you dont want to kill the batteries by discharging past the 50% mark i truly dont understand how you can say It would be adequate for charging a battery no larger than 60 Amp/hrs (12 Volt).

    Is this not about consumption and then battery size then how do we keep up with that consumption replace it OR run whatever off the solar with the batteries there for times when we dont have solar - like when itisdarkoclock:blush::D

    truly not looking for a fight or an argument but to learn and understand.

    thanks in anticipation

    David
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: How do I know amp hours going to battery?

    Good point, David; I should always include my suspect math.

    First point; most of us can't count on 9 hours of "good sun equivalent". The averaged figure is 4 hours. Such pessimism pays when you're having to depend on solar power. :p

    So the basic math goes something like this:

    55 Watt panel operating @ 80% efficiency = 44 Watts over 4 hours = 176 Watt hours.
    See? Somewhere between calculating and typing the response 116 Watt hours vanished! :blush:
    I blame heat; it's 34 C here today and we all know heat has an adverse effect on solar power generation. :p

    So I probably did the other calc wrong as well. Let's see:

    55 Watt panel @ 80% = 44 Watts divided by 14.2 Volts charging for "12 V" system = roughly 3 Amps current. Multiply by 20 (5% minimum charge rate) = 60 Amp/hr battery. Nope, got that one right.

    Well 50% success isn't bad.
    Actually it is. :blush:
  • Highland_Fling
    Highland_Fling Solar Expert Posts: 37
    Re: How do I know amp hours going to battery?
    Good point, David; I should always include my suspect math.

    55 Watt panel @ 80% = 44 Watts divided by 14.2 Volts charging for "12 V" system = roughly 3 Amps current. Multiply by 20 (5% minimum charge rate) = 60 Amp/hr battery. Nope, got that one right.

    Well 50% success isn't bad.
    Actually it is. :blush:

    OK so you are saying you will input to the battery or output from the controller 3 Amps for 4 hours = 12A/H

    And you are saying that 3 Amps is the minimum charge rate for a 60A/H battery...........................

    BUT surely you have not considered the consumption (not that we know anything more than the basics {one 55W panel} ) if you can only generate 12A/H/D and you consume 14A/H/D OR MORE :grr surely you are in deep poo

    What is wrong with my first proposal work out your consumption then size your batteries then size your panels/controller to support that consumption with some headroom for overconsumption and not so wonderful sunny days.

    The batteries need to be 50% or more bigger A/H wise than your calculated/planned daily consumption and the solar needs to be able to support the consumption AND charge the batteries in the morning to catch up with overnight consumption.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: How do I know amp hours going to battery?

    Yes, indeed: charge rate minus usage equals effective charge rate. This is where people often come up short with the 5% minimum (and why I recommend targeting 10%).

    It's not all about replacing the Amp hours "used"; it's also about replacing them at the proper rate for the batteries. Don't confuse my 176 Watt hours of potential panel 'harvest' with usage capacity: 176 Watt hours on a 12 Volt system is roughly 14 Amp/hours.

    So you have two things: one, you must "put back" the Amp/hrs "used", which requires more energy than taken out due to charging inefficiencies (usually 20-25% more). And two, you have to ensure a sufficient charge rate for the battery to remix electrolyte. These two things don't always go hand-in-hand, as in if you discharge the battery too deeply and there's not enough sun hours (equivalent) to put it all back, even though you may achieve the desired rate of charge.

    If you have a 200 Amp/hr bank and discharge it to 50%, you could put back that 100 Amp/hrs in 60 hours with a 2 Amp charger (120 Amp/hrs divided by 2 Amps) in theory. In practice you'll be buying new batteries pretty soon because the old ones are sulphated scrap metal.

    By the same token, you'd want to put a minimum of 10 Amps to that battery to prevent this problem. But if you're drawing off three, you're only really charging with seven. Likewise, if your 10 Amps only lasts for five seconds your batteries are going to be ... sulphated scrap metal.

    But if you can get that target 10%, 20 Amp charge rate and maybe hold its equivalent for five hours ... you've solved both problems.

    Tricky, isn't it? This is why we say you can never have too much panel!

    As I said, that 55 Watt panel is small. In terms of current it could at best handle a 60 Amp/hr battery. In terms of potential harvest it would, on average, put back 14 Amp/hrs maybe 15. Curiously, that's about 25% of a 60 Amp/hr battery and you will often see recommendation of not exceeding 25% DOD mentioned around here.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How do I know amp hours going to battery?

    This is getting back to some rough rules of thumb when designing the battery charging system...

    For flooded cell lead acid batteries, 5% (of the 20 Hour Capacity Rating) is the minimum recommended charging current by several battery manufacturers... At a minimum it gets enough bubbling in the battery cell to "mix the electrolyte" while charging/equalizing.

    Also, as flooded cells get old (especcially heavy equipment cells like for forklifts)--their self discharge can be as high as 1-2% per day--And less than 5% charge rate for 4-8 hours per day will just about keep an old battery float charging.

    It also works out for many people, about the minimum amount of charging current to count on using solar panels to power a "small" set of loads.

    The ~13% maximum charging also comes from flooded cell manufacturers--Above this charging current it is possible to overheat a battery bank--and also the higher the charging current, the less efficient the charging becomes.

    For AGMs--They both have a lower self discharge rate, no "free" electrolyte to mix/stratify, and many can withstand higher charge/discharge rates (vs flooded cell batteries). So, the 5-13% rule of thumb can be widened--but it still works out nice for sizing the battery/array/loads--Usually works out to be a good cost effective compromise.

    So--when sizing a solar power system--we typically size the battery bank first (daily power usage * 3 days of no-sun * 1/50% maximum discharge = ~6x daily load).

    And then we size the solar array to 1. meet the daily loads (with a derating factor/efficiency derating) and based on how much sun you get (winter/summer "hours of sun" per day)... If we have enough data--normally that works out about to 9 months of solar and the other 3 months with generator assist as needed for loads and weather).

    What sometimes confuses me with poor memory--Opening a new thread without some of the information from earlier threads. Many times, I would just suggest keeping "your system" questions in one thread so that it is easier for us to go back and look at has already been discussed.

    And opening a new thread for "unrelated" system (new system, something different) type questions.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: How do I know amp hours going to battery?

    Ok. I truly appreciate the responses but it sure looks that unless I purchase a monitor, I can't use the multimeter to give me an actual reading.

    If it makes any difference, I'm trying to charge a Group 27 battery (110 amp hours) for use on a popup camper. I have two batteries. The idea is to get a solid 3-4 days use out of a battery, then swap out, and charge the other one for 3-4 days (I have minimal use --- no electronics, just minimal use of water pump, heater fan, and lights (sparingly) and portable radio (and the 'phantom draw' of the propane detector). I'll disconnect battery from camper better part of a day.
    We're 'real' campers --- out hiking, not sitting around watching the tele, computer, or running a nuker.

    It looks like, conservatively, I can get 12 amp hours a day. I understand the arithmetic, but it's all estimates. I want to see what is actually happening, while charging.

    But again, will my multimeter show me anything?


    Thanks again for responses. My little panel should be enough (with two batteries) to keep me going.
  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    Re: How do I know amp hours going to battery?

    Regarding measuring current with a DMM:
    As BB. pointed out in Post #2, you can use a DMM to measure the current into or out of a battery. Your panel probably won't put out more than 4 amps so you could use your DMM to measure amps into the battery.

    But your question is "How do I know amp hours going to battery?"

    To do this you need to measure not only the amps into the battery but also the length of time that those amps are flowing. You could take periodic current measurements over a certain length of time, average the amp readings and multiply that result by the length of time that you took the measurements. This would be an approximation.

    Or you could look at some of the devices offered by this vendor:

    RC Electronics
    ... My little panel should be enough (with two batteries) to keep me going.

    I wouldn't be so sure. That's about a 3% charge rate (5% to 13% is recommended.) Also, swapping batteries is a pain (I've lived that way with AGMs) and with FLAs it can be dangerous too.

    K
  • Jakachira
    Jakachira Solar Expert Posts: 47 ✭✭
    Re: How do I know amp hours going to battery?

    I simply do not wan't to underestimate solar power system

    Your solar panel is rated 55Watts. If you crunch the numbers of the solar power it gives per hour in full sun and at low temps, you will notice that it actually surpasses 55Watts an hour.

    I have a 100Watt panel, which gives 6.7amps @ 21Volts in full sun. Which is 130.7Watts per hour. 30Watts more than the rated output of the panel. Which means all the efficiencies are already taken care of in this case. So in clear skies you wouldn't suffer heavy loses of 50%. I notice only about 10% loses, that is to say i see 90Watts (not 50Watts) going through to my battery per hour.

    You have 9 hours of sun, i also have 9hours of sunshine per, The maximum my system get is 15.5Amps. The minimum being 10.1Amps at 8am and 5pm. My evarage is 12.5Amps x 8 = 100Amps per day. This explains it because i have a 105Ah battery which gets around 90% charge by afternoon. At 3pm, it will be full. I only discharge 50% of it, and 80% sometimes.

    All, i'm saying is my system is exceeding the numbers being calculated here. (50Watts x 4 = 200Watts way to below what the systems gives). Probably conditions differ

    I would expect about 40Watts to 45Watts going into the battery per hour with your panel. In a clear day, you would get about 300Watts (with no loading during the day). Real-time monitoring can be best achieved with NASA Battery Monitor..
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: How do I know amp hours going to battery?
    Probably conditions differ

    There you go. Solar is highly site specific. When we talk about 4 hours of "good sun equivalent" that's a year-round, latitude-general figure. It does not pay to think you will get the "nameplate" rating of any panel, especially not all the time. If it appears you are getting consistent results significantly above ratings you should check the accuracy of whatever you're measuring your output with. Never assume it is correct.

    When you're close to the equator you have more "good sun equivalent" time, but usually higher panel temps as well. Move North, and the factors begin to inverse.

    I think I'll get me one of them over-priced dual-axis panel trackers. That way in the 16-hour Summer days here at 3000+ ft altitude under clear blue skies and cool temps I can produce daily kilowatt figures that will blow everybody's minds! :p

    Of course when Winter comes and there's barely two hours of sun a day they still won't put out squiddle, even if it is -40.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How do I know amp hours going to battery?

    There may also be another explanation...

    There is Vmp (Voltage maximum power) and there is Voc (Voltage open circuit). There is also Imp (Current maximum power) and Isc (Current short circuit).

    If you only have one DMM (digital multi-meter), if you first measure the voltage from the panel with no load (Voc) and then measure the output current (Vmp or Vbatt) through the DMM, your Power "math" will not be correct (or if you measure output current from the panel directly to a current meter--this is a short circuit or Isc):
    • Power = Volts * Amps
    • Pmp = Vmp * Imp
    • Pmp /= Voc * Isc
    Generally, a panel will not produce much more than maximum rated power unless you are in a very cold climate and have lots of reflected light from a snow field.

    I believe you are in Malaysia and I would not expect more than rated power (and really only 80-85%) of rated output unless you have reflected sunlight "helping" the panel (once panel has been out in sun heating for a while).

    In any case, even if you did Voc cool and used Isc--I would expect "calculated" maximum power to be around 100 Watts maximum for a 55 watt panel.

    Either you are doing something we don't understand, the panel is actually under rated, or (possibly) you are using the current measurement from the solar charge controller (solar charge controllers are know to read from 5% to as high as 30% high over actual performance numbers--even very good controllers--most are not designed to give 100% accurate current readings--it is just not needed for proper operation and can save a few watts of energy and some mfg. costs).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: How do I know amp hours going to battery?

    stc pv ratings also are being given at 25 degrees c and pv outputs slightly degrade above that and slightly build below that. unfortunately, pvs aren't at the ambient air temperature as the solar radiation heats them up far above ambient.
    also note that just having daylight is not having the full solar intensity that pvs are rated at for all of those hours. that rating is what i term as a full sun as the intensity is at the 1000w/m^2 and i can guarantee that you aren't getting that level early in the morning or late in the evening and you might get it around solar noon. the odd times are not without merit as 2 hours at half intensity is equal to one hour at full intensity and hence we term it as a smoothed reference to full sun hours or the equivalent of full intensity hours of sun one receives in a day. it usually is about 5-6hrs a day in our summertime and where you are it may very well be like that year round being nearer the equator. this does not account for clouds and rain you may receive and many places do give data for area specific figures.
    the guys are also correct in that you can't multiply the isc and voc together to get pv ratings. it is imp x vmp to get wmp and the stc conditions at which these are measured are more rare and unrealistic for most of us. max power only occurs at the max power point as open circuit voltages have no current and 0 x vmp = 0w power. same with the isc rating as a short theoretically brings the voltage to 0 and isc x 0v = 0w power.
  • BilljustBill
    BilljustBill Solar Expert Posts: 219 ✭✭✭
    Re: How do I know amp hours going to battery?
    I have a new solar setup. 55w solar panel, charge controller, inverter, all set up.

    I have measured voltage on panel in the 18volt+ range --- all ok.

    I simply don't know how to use the multimeter to determine how much I am charging the battery. Of course, I can measure the battery at the end of the day and compare readings before charging, but can I see how much I'm actually charging in 'real time' and know how many amp-hours I'm replacing in the battery?

    I realize the sun will come and go, angle changes, etc. Just want to know, at a given point, how much charging I'm getting and to change angle of panel.

    Thanks. I know this will be simple.

    Would this serve your needs?

    http://store.solar-electric.com/tm2020.html

    Bill
    Bill
  • cfcw
    cfcw Solar Expert Posts: 25
    Re: How do I know amp hours going to battery?

    I experiment with small panels and highly recommend the Watts Up or Doc Watson meters produced by RC electronics. They will allow you to learn a lot about real-world solar charging and usage of power. They give instant feedback, so you can use them to optimize your panels orientation to the sun if you wish. Turnigy makes a cheaper one, but is not as accurate. My Turnigy reads 350 ma low.
  • retrodog
    retrodog Solar Expert Posts: 53 ✭✭
    Re: How do I know amp hours going to battery?
    BB. wrote: »
    Assuming your solar charger does not have a current meter:
    • Battery Monitor--with a "shunt" in the negative battery lead, designed to keep track of Amp*Hours of current flow in/out of battery bank to give an estimate of the current State of Charge of the battery bank. Bi-directional measurements (add/subtract from master total).
    • Amp*Hour / Watt*Hour meter--frequently used by RC (radio control) and other hobbyists to measure voltage, current, current*time, and watts*time of flow to or from a battery (one way only for these models.
    • Current Shunt and millivolt meter... Place a precision resistor in battery negative leg and use a volt meter to display current flow (shunt is exactly the same part used with a Battery Monitor).
    • If the current flow is less than 10 amps maximum, you can put a DMM in the negative lead of the battery. Note--if you do this wrong, it is very easy to destroy your DMM or worse.
    And to address another unasked question--To measure AC power / kWH--a Kill-a-Watt meter is a great piece of equipment to use around the home (find those energy hogs).

    And for measuring whole home AC power usage--there are various units, like the T.E.D. for doing that. Very nice if you have large stuff (electric stove, well pump, A/C, etc.).

    The above are just starting points for you to take a look at. There are many options out there and some may be better for your needs than others.

    Please feel free to ask more questions.

    -Bill

    I'm glad that I caught this post cause it made me remember about what I originally wanted to do. Frys has a wide assortment of panel mount analog meters from Velleman and they are only $6.99 each:

    http://www.frys.com/search?search_type=regular&sqxts=1&query_string=velleman+meter&submit.x=0&submit.y=0&cat=0

    I hope that link works.

    Anyway, it should be easy to build up a multiple meter panel to show all the important stuff at a glance.
  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    Re: How do I know amp hours going to battery?
    retrodog wrote: »
    I'm glad that I caught this post cause it made me remember about what I originally wanted to do. Frys has a wide assortment of panel mount analog meters from Velleman and they are only $6.99 each:

    http://www.frys.com/search?search_type=regular&sqxts=1&query_string=velleman+meter&submit.x=0&submit.y=0&cat=0

    I hope that link works.

    Anyway, it should be easy to build up a multiple meter panel to show all the important stuff at a glance.

    They also make a 30A model with current shunt. Cheaply made, not very accurate but it works as a general indicator. http://www.parts-express.com/pe/showdetl.cfm?Partnumber=320-233

    It's good value because a good Simpson panel meter is 6X the price. http://www.alliedelec.com/search/productdetail.aspx?SKU=9185123
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: How do I know amp hours going to battery?

    My next plan is to get an old style, add-on automotive amp meter, in the 100A range.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Highland_Fling
    Highland_Fling Solar Expert Posts: 37
    Re: How do I know amp hours going to battery?
    mike90045 wrote: »
    My next plan is to get an old style, add-on automotive amp meter, in the 100A range.

    I dont know if this is allowed but i can get you a UK car - 30 +30 and -60 - +60 2" Smiths ammeter for around $40 plus shipping from the UK PM me if anyone is interested - i have a Paypal account :D:blush::-)
  • Highland_Fling
    Highland_Fling Solar Expert Posts: 37
    Re: How do I know amp hours going to battery?

    Now i want to look at real time figures historically so i can evaluate how well the panels are performing on any particular hour/day.

    I know a Xantrex Link Pro Battery monitor wont do that - not give me the detail i 'initially' want to examine yes it will give me totals but i want more than that certainly until i get to know my system - after that - then of course it will just be boring data to baffle fellow sailors with :p

    Will a Morningstar MPPT 15L - Tristar 45 and Tristar 60 do that - i do have a laptop to look at these figures.

    OR do i need a Bogart Engineering Trimetric or Pentrameteric unit?

    For sure i am not going to sit watching these figures on an interactive basis I want to examine what has been going on at my leisure and when i have the time.

    thanks again

    David
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: How do I know amp hours going to battery?
    I dont know if this is allowed but i can get you a UK car - 30 +30 and -60 - +60 2" Smiths ammeter for around $40 plus shipping from the UK PM me if anyone is interested - i have a Paypal account :D:blush::-)

    I'll get it localy, with it's own shunt. Thanks though.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Jakachira
    Jakachira Solar Expert Posts: 47 ✭✭
    Re: How do I know amp hours going to battery?

    Steca Solar Regulator, what a cool item this one is

    This is what it can do for you

    1) Displays SOC in % (0-100) - this is only valid when there is nothing connected to the battery during charging

    2) Displays Voltage of the battery if something is connected directly to battery, thus monitoring is done using voltage

    3) Displays Module Current real time

    4) Displays Total Ah put into the battery from the very time you start charging your battery (I like this you can actually see that i have put say 30a into the battery so far)

    5) Displays Total Ah discharged through the load terminal. (if you have 12Volts items you can see the current you have used since you connected the load)

    6) Displays Total Ah generated since you installed the system. its different from number 4 because number for is for a specific charging. This one is since you installed the regulator

    7) Automatic lighting in the night.. you can set reg to auto switch on your lights at night and off in the morning.

    8) Automatic battery equalisation

    What an item, here in south Africa, the Steca PR30 (12V, 30A max) is only R1500.00 (US$160.00).