Temperature compensation need?

ShadowcatcherShadowcatcher Solar Expert Posts: 228 ✭✭✭
Newcomer questions, from someone who did not do their homework adequately to begin with!:roll:
This installation is going into a teardrop trailer that has been set up for boondocking.
AGM battery, all LED lights, 12V water pump and filters so that you can pull water from streams and lakes safely…
I have a 180 W solar panel that evidently was meant for a grid tie system (my mistake) as it is putting out over 40 V under load. My efforts to find a DC to DC converter that would get the voltage down to what the Sunsei CC20000D controller would handle, 24 V, were unsuccessful. So I am ordering a Morningstar SS MPPT 15L, remote meter and meter bus.
My questions are, do I need the temperature compensation probe with the AGM battery. Can I hook up an additional 20 W panel in series, different voltage?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,997 admin
    Re: Temperature compensation need?
    I have a 180 W solar panel that evidently was meant for a grid tie system (my mistake) as it is putting out over 40 V under load. My efforts to find a DC to DC converter that would get the voltage down to what the Sunsei CC20000D controller would handle, 24 V, were unsuccessful. So I am ordering a Morningstar SS MPPT 15L, remote meter and meter bus.
    Good choice. The MS MPPT 15 amp controller is very nice.
    My questions are, do I need the temperature compensation probe with the AGM battery. Can I hook up an additional 20 W panel in series, different voltage?
    AGM batteries are very sensitive to over charging (cold controller/hot battery could be an issue) and the MS MPPT 15 amp controller really "must have" the RBTS (remote battery temperature sensor) option as it tends to over estimate the battery temperature (causes lower output voltage which slows battery charging).

    I would humbly suggest getting a Battery Monitor so you can watch the state of charge of your battery bank.

    AGM (and other sealed batteries) do not allow you to measure specific gravity--and you are left to measuring the battery resting voltage (2-3 hours of no charging/load currents) to estimate the state of charge of the battery. (Battery monitors are not perfect--but they do a pretty good job of letting you know if you are discharging the battery too much and/or not recharging it properly).

    Here is a nice thread with video from Keven in Calgary Canada that shows designing and installing solar PV in a small RV trailer installing a very similar setup as yours.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,997 admin
    Re: Temperature compensation need?

    Forgot to add a comment about the 20 watt panel...

    You can add panels in series if their current is within ~10% of each other (i.e., 4 amps for one, 3.6 to 4.4 amps for the second panel).

    Similar for adding in parallel... Vmp should match within ~10%.

    If the panels are too different--you need a second charge controller to connect the second panel/array to your battery bank.

    And, of course, the Vmp/Voc/Imp/Isc ratings of the array must not exceed the input ratings of the charge controller.

    In your case, if your Sunsei CC20000D controller is still good--you could use it on the 20 watt panel (to be honest--adding a 20 watt panel to a 180 watt panel is, almost, not going to make any difference).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Temperature compensation need?

    According to Windsun's battery FAQ its the gel-cells that need to be handled with care. The AGMs can take some abuse like overcharging.

    http://www.windsun.com/Batteries/Battery_FAQ.htm#AGM,%20or%20Absorbed%20Glass%20Mat%20Batteries
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • BB.BB. Super Moderators, Administrators Posts: 32,997 admin
    Re: Temperature compensation need?

    Given that I am no battery expert--AGM's can take more abuse than Gel Cells... However, if you overheat them and cause venting--there is no standard was of restoring the electrolyte to the cell.

    Also, AGM's use a small catalyst in the cap that recombines the H2+O2 back into water... That also generates heat. All adding to the warning about overcharging/equalization of AGM batteries.

    With flooded cell batteries, they just vent the gasses (no heat from recombination of gasses) so there is (as I would guess) less heat generated in the battery itself and you can (obviously) add distilled water to make up for fluid losses.

    However, overcharging / heavy equalization is still hard on any type of lead acid battery. It erodes the plates and drives oxygen into the positive plate causing corrosion (case swelling and pushing up the positive posts over time).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Temperature compensation need?

    Heat in the AGMs is mentioned in the FAQ, basically that it isn't a problem. Quote:

    "...since the internal resistance is extremely low, there is almost no heating of the battery even under heavy charge and discharge currents. The Concorde (and most AGM) batteries have no charge or discharge current limits."
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • BB.BB. Super Moderators, Administrators Posts: 32,997 admin
    Re: Temperature compensation need?

    I understand the current limit is very high (upwards of C*4)... However, overcharging (once the battery is 100% charged) is an issue.

    For example:
    Overcharging: Especially fatal to Gel and AGM batteries, consistent overcharging (NOT equalization) boils the electrolyte out of the cells, and can even lead to thermal runaway, with the battery becoming hotter and hotter. One of our writers experienced thermal runaway on his liveaboard Catalina 30, caused by a ferroresonant "dumb" charger, with nearly catastrophic results.

    And:
    While our VRLA batteries accept a charge extremely well due to their low internal resistance, any battery will be damaged by continual under-or overcharging. Capacity is reduced and life is shortened.
    Overcharging is especially harmful to any VRLA battery because of the sealed design. Overcharging dries out the electrolyte by driving the oxygen and hydrogen out of the battery through the pressure relief valves. Performance and life are [r]educed.
    If a battery is continually undercharged, a power-robbing layer of sulfate will build up on the positive plate, which acts as a barrier to recharging. Premature plate shedding can also occur. Performance is reduced and life is shortened.
    Therefore, it is critical that a charger be used that limits voltage. The charger must be temperature-compensated to prevent under or over discharging due to ambient temperature changes.

    Concorde Battery Manual (PDF):
    Gel batteries are also more sensitive to charging voltage. If the charging voltage is not controlled within a very tight range relative to the battery’s temperature, the life of the battery will be adversely affected. For example, one manufacturer of gel batteries claims that if the charging voltage is 0.7V higher than the recommended level, the cycle life will be reduced by 60 percent. The reason for this effect is the limited oxygen recombination capability of gelled batteries. Lifeline® AGM batteries are more forgiving in overcharge conditions and their ability to recombine the hydrogen and oxygen gases back into water is more efficient. With Lifeline® AGM batteries, tests have shown that increasing the charging voltage 1.0V above the recommended charging voltage results in only a 23% reduction in the cycle life.
    ...
    Sensitivity to Charging Voltage Levels
    • AGM: Moderately sensitive. Life is somewhat reduced if charged outside of recommended charge voltage levels.
    • Gel Cell: Very Sensitive. Life is greatly reduced if charged outside of recommended charge voltage levels.
    ...
    Charging Lifeline® AGM batteries is a matter of replacing the energy removed during discharge plus a little extra to make up for charging inefficiency. The amount of energy necessary for complete recharge depends on the depth of discharge, rate of recharge, and temperature. Typically, between 102% and 110% of the discharged ampere-hours must be returned for full recharge.

    The most efficient method of charging Lifeline® AGM batteries is to use a 3 stage charging profile. In the first stage, a constant current is applied until the voltage reaches a pre-set limit. The first stage is often called the Bulk charging stage. In the second stage, the voltage is held constant at the same pre-set limit until the charging current tapers to a very low value, at which point the battery is fully charged. The second stage is often called the Absorption charging stage. A voltage limit of 14.3 volts ± 0.1 volts (7.15 ± 0.05 volt for a 6 volt battery) should be used when the battery temperature is 77°F (25°C). The battery is fully charged when the current drops below 0.5% of the battery’s rated capacity (0.5A for a 100Ah battery). In the third stage, the charging voltage is reduced to a lower value that minimizes the amount of overcharge, while maintaining the battery at 100% state of charge. This third stage is often called the Float charging stage. A float voltage of 13.3 ± 0.1 volts (6.65 ± 0.05 volts for a 6 volt battery) should be used when the battery temperature is 77°F (25°C). The charging voltages at other temperatures can be determined from the following table:

    Not saying that a little overcharging will kill an AGM battery immediately--but constant over voltage will shorten their life (partially, because water cannot be added back into the cell).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ShadowcatcherShadowcatcher Solar Expert Posts: 228 ✭✭✭
    Re: Temperature compensation need?

    I will be using a Lifeline battery (Concorde's RV/marine AGM) and when I spected the battery I knew that it would be used with a Progressive Dynamics power center/ converter that has an equalize mode of 14.4 V every 21 hours for a period of 16 min. I contacted the folks at Concord and asked if this might be a problem. The engineer that responded indicated that they love the Progressive Dynamics converters because they assure that the battery is fully charged during that equalize mode.

    I have ordered the temperature sender, figuring that is just good insurance in long battery life. The teardrop will probably spend more of its time hooked up to shore power than not but there are times particularly when we are traveling long distances or camping on Crown land in Canada or BLM land in the US that I want to be relatively self-sufficient.

    Part of my nature is that I want to know why something works, or doesn't to the point of being a bit obsessive about it and appreciate your help.
  • KamalaKamala Solar Expert Posts: 452 ✭✭
    Re: Temperature compensation need?

    Shadowcatcher,

    Good luck on your endeavor! I won't attempt a detailed analysis of your system. Instead, I'll just share my (limited) experience and knowledge regarding AGM batteries.

    I chose Concorde/Lifeline in my original design. I did not find a local distributor that had them in stock. I wanted to avoid the shipping charges. So I "settled" for East Penn 8AGC2. Sold under many names: Sunextender, MK and Deka are a few. (Kinda like photographic lenses.) My batteries have a sticker on them that says Northern Battery, my local distributor. That distributor was forthcoming enough to point me toward the actual manufacturer. So I contacted them and got great specs on the charge profile for my battery.

    IMHO temperature compensation of any AGM charger is mandatory! The charge voltage must be stringently controlled. One has more leeway with current. But just a little.

    The reason I chose AGMs is that I cannot tolerate any out gassing since the batteries are in my camper. Under the bed!

    Keep an AGM charged under the proper constraints - no problems.

    good luck hth

    k
Sign In or Register to comment.