Charging Current to Battery Bank?!?

tkc100
tkc100 Solar Expert Posts: 67 ✭✭✭✭
Currently my solar panels are down and I have to live off a generator. With the cost of fuel and the wear and tear on the generator I would like to use it as little as possible by using my battery bank. Likewise I would like to get the batteries from a 50% SOC to fully charged as quickly as possible.
Please take a look at the attached flow chart. Has anyone done anything like this before?
Are there problems with this type configuration?

I do have still yet another question I can’t seem to resolve. The actual current output of the power supply is not known. It could be as high as 50 amps. I know there is an inverse proportional relationship between voltage and amperage in a lead acid battery. In a charge cycle amperage is high and voltage in low. As the battery charges the voltage climbs and the amperage the battery will accept declines. But in a worst case scenario with a fully discharged battery won’t my proposed setup dump all the current the power supply can deliver and there by damage the battery? Or still yet would the battery overload the power supply and smoke it?

You know I am a good tinker and can build most anything but sometimes I get my mind in a twist and need some help straightening it out.
Any assistance will be greatly appreciated.
Thanks!

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Charging Current to Battery Bank?!?

    Batteries have near zero resistance... So your DC Current/Voltage source needs to be able to no overheat/over-current when the batteries are low...

    And the DC regulator needs not to over voltage or overshoot output voltage maximum when the charge controller goes into PWM mode. (if this is a "simple" transformer and diode bridge--your transformer may have some pretty good inductive kick.

    The other issue with a simple Transformer/Diode bridge--Most of the current is right at the peak of the AC sine wave. This can cause the RMS current to go up quite a bit and overheat the diodes, transformer, and/or even the genset (heating is I^2 * R--So if the peak current is doubled, the waste heat goes up by 2*2 or 4x).

    People have done your design before--with some modifications... The Battery Charger is replaced with a 24 or 48 volt battery bank, and the PWM C60 type charger is replaced with a MPPT type charge controller (MorningStar 15 amp MPPT for sure).

    But--that sort of is a waste of money for you--it would be cheaper to get an Iota battery charger or just use an automotive battery charger and unplug it when the batteries are full.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • samuel
    samuel Solar Expert Posts: 80 ✭✭✭✭
    Re: Charging Current to Battery Bank?!?

    While not a direct answer to your question, one option to conserve fuel could be to leverage appliance use with battery recharge. While I'm not quite clear on how your system is set up, something that may help is to run the generator when you are using the most power (when lights, TV, well pump, other appliances are in highest use) so that excess not captured by the batteries is available for normal appliance use.

    For example, I believe the EU2000i idles down to around 500 watts on eco throttle, so if the batteries are nearing float and only accepting a 1/4 of a 1/2 of 500 watts the extra could be used to power appliances instead of being wasted.

    This worked for a time at my families cabin until we could afford to put the solar in (and when the ground thawed out). I found that when the batteries require a charge, night time was best due to TV, lights, and other appliance use.
  • tkc100
    tkc100 Solar Expert Posts: 67 ✭✭✭✭
    Re: Charging Current to Battery Bank?!?

    BB
    Thanks for input!
    Another one of the things that seems simple on the surface but in reality it’s not. I’ve certainly got an education on this one and hopefully can remember all I have learned for a long enough period of time to make use of it.
    I am currently living off the generator and it fed into a converter. The converter has at its heart a ferroresonant transformer which makes a good stable power supply (12 volts) but a lousy charger. (See attachment)
    I have got a C-60 controller and two different power transformers, a 16 and a 24 volt. I thought it might be a simple matter to change out the transformers. This would give me adequate voltage to charge a lead acid battery and in series with the C-60 I would have a three stage charger.
    Well when I opened up the converter things started going south. The ferroresonant transformer used a center tapped full wave rectifier bridge. Not bad less than $25 to correct. (See attachment)
    It was about this time I started to attempt to remember some past lessons learned. I wondered what would control the current flow to the batteries. Both transformers are rated at around 50 amps, too much for my batteries and if the batteries were near dead I feared for the life of the power supply.
    Also there was a great dark spot in my knowledge and that was how the C-60 would perform in this configuration.
    So in your opinion and by the way I can find no fault with it, all the parts including the converter (now in parts) are nothing more the spare parts.
    I need to start saving up the $$ for a charger. Do you think Iota is the way to go? I am doing the battery charger thing now and it’s a real pain. Many if not most automotive battery charger top out at 15 to 16 volts. I know there are some better more sophisticated chargers out there now but mine is not one of them and 16 volts is really not healthy for a lot of other circuits. So I have to plug it in and constantly monitor the voltage. Not long before 200 or so dollar doesn’t sound like so much.
    Sometime when you’ve got lots of time I will tell you about the cheap fishing boat I bought.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Charging Current to Battery Bank?!?

    I have done my fair share of magic smoke projects (plug in and see what happens)... It is very difficult for me to really make any helpful suggestions through a web forum that would give you "safe" answers to how to test/modify for your needs.

    If this was you and your garage with a trolling motor battery--Why not try and see what happens. Usually, the 16.x volts is an RMS reading ("average power" voltage)... For sine waves that is 1/srt(2) or ~0.707 -- Or the DC peak voltage is ~1.414x higher than the RMS voltage -- So you will still have charging current flowing (less than full current) up to Voc of ~16.4v*1.414=23.2volts...

    So--you should be able to manual charge the bank...

    But given that batteries are temperature sensitive (do you have hot/cold conditions?)--And you want other family members to safely charge the battery bank/run the genset without you being stuck there for hours to keep things safe and working...Yea, $200 (or more) for a bullet proof battery charger starts to make sense.

    In this thread (anyway)--you have not gone into details about genset model/size and battery bank capacities... I am a big believer in matching sizes to cost effective operation--As Samuel says--if you can run your genset at 50% or greater of rated load--You can run some of the inverter type gensets (Honda eu000i family, and many folks like the Yamaha inverter gensets too), with fairly good fuel efficiency down to 1/4 of rated load...

    Matching genset to battery charger to battery bank can really save some money (i.e., don't use a "cheap" 5kW genset to run a 50 amp 12 volt battery charger (less than 1,000 VA) unless you have other power needs too (i.e., run the irrigation/cistern pump while charging your battery bank).

    For a smaller genset/system--you might look at getting a Kill-a-Watt type meter so you can properly load the genset and also log kWH generated vs gallons of gas used (i.e., kWH per Gallon).

    I keep pointing people with small genset/battery charger questions to this thread by SteveK... It really goes into the details of picking the "optimum" setup and why it can be a good thing.

    Question about battery charger selection with EU2000 generator

    Sorry to be repetitive about the above link... But it does answer a lot of questions.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dr. Strangelove
    Dr. Strangelove Solar Expert Posts: 49
    Re: Charging Current to Battery Bank?!?

    Just as an aside, a ferroresonant transformer is not very efficient. This is because it operates by saturating the core - in a non-linear region - so that additional current produces no further flux coupling. The problem then is that this power is lost. Ferroresonants are great for regulating output voltage from input voltage, but they produce a nearly square wave output. I wouldn't recommend them for a RE design.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Charging Current to Battery Bank?!?

    i think i'll go into a little bit of general speak here on this subject and kick it around from my perspective. the problem with listing a power supply to feed a controller is that the controller when bulk charging is wide open to sending all of the current available up to the limit of the controller. in your case this is 60a. the power supply will essentially see no controller while bulk charging. herein lies the problem. most power supplies when connected to a battery will want to send a near infinite amount of current as the low internal resistance of a battery will seem like a dead short to the power supply. that power supply has to have some way to limit the current or it will pop. few power supplies have current limiters built in as most are geared to just regulating the voltage. there are good power supplies out there that do have current limiters and some are even adjustable. big $ for good quality older linear power supplies. not sure of some of them out there today, but current limiting would be a must.

    moving on to the subject of the battery chargers, the real pains are the ones that look for the battery voltage aka microprocessor controlled types and yes, they put the voltage at too high of an amount. i tried using a diode on one i purchased, but that blocked the presence of the battery voltage and it will not charge. now i did not try 2 in series with one reversed in parallel across the series diodes to see if that had the desired effect and theoretically this could possibly fool the charger into thinking it is roughly about .5v or .6v higher than it actually is. again, i didn't go that far.

    the older type automotive chargers can be used up to a cc with success if the output of the charger is capacitively filtered enough to reduce the ac ripple being introduced to the controller input. i used a 6/2 schumacher (spelling) charger with a large electrolytic computer cap of around 30,000mfd and fed it into a morningstar sunsaver 10 pwm cc. this worked fine until there was a failure in the charger itself as the transformer in the charger shorted internally and sent 125vac down to the cc and blew it out. lucky for me i was present when this happened so this is very risky to do and i would never leave such an arrangement in place for long or unattended. i am lucky too that the electrolytic cap did not explode as i pulled the leads in time i guess.:-)
  • 2manytoyz
    2manytoyz Solar Expert Posts: 373 ✭✭✭
    Re: Charging Current to Battery Bank?!?

    Iota chargers are pretty decent for the price. Very popular with the RV crowd. I had one in my 36' Class A motorhome.

    I purchased a 75A Iota charger used from Ebay. Got it for ~$100. Works great with my 900 AH battery bank (8 6V T-105 type batts). I primarily only use this when the weather is bad and the solar array isn't up to par, or after a major outage. I have custom built single circuit automatic transfer switches on appliances, allowing them to operate from an inverter when the grid is down. I plan on buying a different home in a year or two, so I didn't bother with a large transfer switch. These go between the appliance, and the outlet.

    If the sun is out the next day, I can use the Iota charger (75A) in parallel with the solar array (50A), to charge at ~ 100-125A (briefly) until the battery voltage starts to increase, and the charge rate starts to taper off.

    dscn4134-1.jpg

    Details about my setup here: http://2manytoyz.com/iotacharger.html

    The instructions for all my charge controllers specifically state not to use them as a regulator for a power supply.

    I'd recommend buying an Iota (type) charger, sized for your setup, and avoid any headaches.
  • tkc100
    tkc100 Solar Expert Posts: 67 ✭✭✭✭
    Re: Charging Current to Battery Bank?!?

    Thanks and many thanks again.
    There is a least one benefit to these mental exercises. I get an education. Now if only I can remember long enough to take advantage of all my new found knowledge. You all have saved me a lot of frustration.
    This project is now and perhaps will be forever ongoing. It’s my own personal adventure with living off the grid.
    I have done a number of other solar project, some with your assistance but those were simpler in that I only had to spec it out and install it. This endless tinkering stuff seems to have no end.