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Renewable Energy: Protecting battery chargers from overvoltage

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  1.  
    • CommentAuthorGBP-Keith
    • CommentTimeAug 4th 2012
     
    Does anyone know of a product or DIY solution that can protect regular 12v or 24v battery chargers from an overvoltage spike from renewables?

    Here is the scenario:
    I have a 2.5 wind turbine and a number of other renewable technologies charging large battery banks. Some energy from some of my generators (hydro and some solar) is added to the batteries via simple chargers. However when the wind blows gusty (often with sunny weather) I occasionally get spikes up to 30volts or even slightly higher. This is OK to some degree as it helps equalize the batteries but I have found that it can (and is) damaging some of the chargers. The automatic (3 stage) chargers seem more vulnerable as they seem unable to keep up with the rapidly changing voltages.

    I thought that some sort of blocking diode would be the answer but one charger manufacturer said that the charger will fail to see the batteries.

    Any ideas please- all suggestions welcome.
    Cheers, Keith
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 4th 2012
     
    Well here's a couple:

    1) Have an clipper/limiter on the turbine, eg as here: http://www.earth.org.uk/wind-power-pilot-autumn-2007-MotorWind.html

    2) Add a large(r)/quick dump load to help keep the battery volts down earlier so you don't get to a stage where the spikes can occur (the battery will absorb them).

    I do both on a small basis with my off-grid system, and size (1) up so that (2) doesn't happen much.

    A variant on (1) is to have it upstream of the rectifiers and/or so it really is only clipping/shorting or a part cycle each time which is a bit kinder to the turbine itself (else it sounds like someone jammed a poker into the blades).

    Rgds

    Damon
    • CommentAuthorCWatters
    • CommentTimeAug 4th 2012
     
    If they are short spikes perhaps something like a 50W 15V zenner diode across the battery would work.
    • CommentAuthorGBP-Keith
    • CommentTimeAug 6th 2012
     
    Thanks both. The problem that I see with 'earlier' dump loading is that it can waste energy (by this I mean put it where it is not actually needed) and thus prevent the batteries from filling completely during the windy period. My experience is that windy periods, certainly in my part of the country are quite short and sharp so to be of use I need to try to catch and 'store' as much as possible. My proven turbine does not use short-circuiting technology to restrain itself but my smaller Air 403 does.

    I'll look into the product solutions that you mention though and report back if they look promising.
    • CommentAuthorEd Davies
    • CommentTimeAug 6th 2012
     
    Posted By: GBP-KeithI thought that some sort of blocking diode would be the answer but one charger manufacturer said that the charger will fail to see the batteries.

    Dunno, but if the charger has battery sense leads separate from the actual charging cables then a blocking diode might improve matters. You'd put the diode in the charge lead and connect the sense leads directly, of course. The charger would still get confused but might be less likely to be damaged.
  6.  
    Posted By: Ed DaviesYou'd put the diode in the charge lead and connect the sense leads directly, of course. The charger would still get confused but might be less likely to be damaged.
    What kind of diode are you thinking of? A rectifier diode will protect against reverse voltage, but only drop 0.6-1V when conducting in the forward direction. A zener diode can dump current to ground, but this wastes energy & would probably interfere with the voltage sensing.

    Posted By: GBP-KeithThe problem that I see with 'earlier' dump loading is that it can waste energy (by this I mean put it where it is not actually needed) and thus prevent the batteries from filling completely during the windy period.
    The only solution which avoids any loss of energy is a switch mode charger which can handle the input voltage swings. What kind of power levels would it need to handle? Do you need to maintain charging when the voltage drops below 12V? Or is the input voltage range essentially 12-30V?

    David
    • CommentAuthorEd Davies
    • CommentTimeAug 6th 2012
     
    Posted By: davidfreeboroughWhat kind of diode are you thinking of? A rectifier diode will protect against reverse voltage, but only drop 0.6-1V when conducting in the forward direction.

    That kind. Possibly less than 0.6 V if a Schottky diode.
  8.  
    How will that help with the over-voltage problem?

    David
    • CommentAuthorEd Davies
    • CommentTimeAug 6th 2012
     
    It wouldn't stop the over-voltage at the battery terminals (which Keith mostly welcomes as a sort of equalization charge) but it would stop it getting back to the more sensitive chargers, at least with any substantial current on the charging terminals.

    If the charger doesn't have separate battery sense leads then the problem would be that, during charging, it'd see the battery voltage plus the forward diode drop so would stop charging early thinking the battery has 0.6 V or whatever more charge than it really has.
  10.  
    The generators connect to the batteries via the chargers. So the charger's power supply input will still see the voltage coming from the generators.

    David
    • CommentAuthorGBP-Keith
    • CommentTimeAug 6th 2012
     
    Interestingly it is switchmode chargers that are handling it the worst. It may be that the model I chose was naff but I have burnt out three of them. The conventional chargers just seem to take it as a knock with the symptoms being a halving of input current across the range.

    The chargers that I am talking about are conventional car/truck chargers from mains quality voltage down to 24v
  12.  
    Keith

    If the input to the chargers is 240V then what's causing the over-voltage? Is the 240V also going over-voltage or just the battery side? Are you trying to combine the outputs of multiple chargers to a common point which feeds multiple batteries? Or are all the chargers feeding separate batteries? Could you draw us a schematic diagram?

    David
    • CommentAuthorEd Davies
    • CommentTimeAug 7th 2012
     
    Posted By: davidfreeboroughThe generators connect to the batteries via the chargers.

    Keith's description is really rather unclear - you're right to ask for a diagram.

    What I was assuming was that the battery voltages were getting too high as a result of high input currents from the wind turbines which was then confusing/damaging the other charge controllers (e.g., for the PV). The diode would have been to protect the charge controllers which were not the source of the problem. From what Keith's just written I think my assumption might have been wrong but I really don't know.

    Keith, is your system AC coupled? That is, with the various generators (wind turbine and PV arrays, etc) feeding 'grid-tie' style inverters on to an 240 V AC bus which is then used for battery charging? I am right in thinking you're completely off-grid, aren't I?
    • CommentAuthorGBP-Keith
    • CommentTimeAug 7th 2012 edited
     
    I have two energy generation systems.

    1. The home is offgrid and is powered from batteries via a Trace 4kw inverter connected to batteries. this system does not have hardwired grid connection.
    2. I also have a grid tied solar PV array that essentially connects direct to the grid independently.

    I charge my batteries from a number of inputs (AC and DC). The two AC inputs relevant to this discussion are:

    A: from a small AC hydro generator which charges through a car battery charger
    B: the standalone Grid tie system (mentioned above) from which I draw a portion of the AC current during sunny conditions only (triggered by contactors) through a another couple of car battery chargers.

    I have three battery banks (2 of which are backup/secondary on dumploads)
    The main battery bank has multiple inputs, as I said including, (depending on the weather conditions) the three (24v) car battery chargers. These all use the same combined route onto the battery bank.
    These AC chargers are the ones that are being disrupted and/or damaged from some unexplained 'occurrence/phenomina/state' in the off-grid system and I suspect overvoltage of the batteries.

    NONE of my DC chargers, PV, wind etc are affected (so i assume protected with inbuilt kit) and at present work perfectly.

    I'll draw a diagram shortly and upload it.
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 7th 2012
     
    I from time to time have multiple lead-acid chargers charging the same batteries, without problem, though most of them explicitly disconnect stuff such as the load if they notice the battery voltage rising too high (>15V for 12V nominal) which makes me think that some are more able than others to (a) void causing the problems and (b) deal with it better.

    Rgds

    Damon

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