battery charger puzzle

[bob golding]

hi all, we are in a calm period at the moment with no  decent wind for a few days,so i am having to keep my batteries charged from the genny. i am not sure of something so looking for advice. the charger is 12 volt 100 amps. the battery bank is 6  110 amp hour FLA batteries. the charger has a variac charge controller. i am only able to get 20 amps into the bank. is this normal? i know how the wind turbine and battery interact, with the voltage being fixed by the battery, but does this happen when using a charger as well. the generator is 5 kw so no shortage of power. when i short the output of the charger it goes up the 100 amps ok. the rectifier is a selenium one. the resistance across the output terminals is around 1.6K ohms. that sounds a bit high to me but i dont know. the charger was in a scrapyard so it could be faulty. anyone got any ideas? should i change the rectifier to a silicon one? i have only had to use it twice in the last 8 months but it would be nice to pump more than 20 amps into the bank when i have to use it.

cheers

bob golding

[electronbaby]

Your batteries aren't heavily sulphated are they?

Are the batteries cold?

If you short the charger and it makes good current, then the problem either lies in a high resistance connection, or in the batteries themselves. right??

what voltage is the charger charging at?

(may be obvious to me, but dont short the charger when connected to the batteries when you are testing tings out)

[bob golding]

charging at 13.8 volts. batteries are cold,and old,but charge fine on the turbine. been googling and find that high resistance is a commom failure mode for selenium rectifiers. still doest explain why it takes full current when shorted though. i wont be shorting the battery bank. been there done that. big bang flash and blew the end off my screwdriver.

[tanner0441]

Hi

If it is a selenium rectifyer and in a scrap yard it has more than likely had its useful life. We used to call them cabbage rectifyers and if you short the charger out for too long, or overload it, you will find out why, a very characteristic smell.

You could change the selenium for a diode bridge but the output voltage will be higher, so you will have to keep your eye on the current.

Brian

[AbyssUnderground]

13.8v... there is your problem. 13.8v is no where near enough to get that battery bank charged. You need to set it to 14.4-14.8v at least. With the increase in voltage you'll see an increase in current as well. You won't see 100A, but you'll see a lot more current than you're currently seeing.

[bob g]

don't you just love the smell of selenium rectifiers in the morning?

some of my earlies memories as a kid building power supplies are of that smell

you get that in your nose and you will forever remember it!

bob g

[Flux]

I agree that the days of selenium rectifiers are over, they were better than the copper oxide but that doesn't say much. Replace with silicon as a first step but even then small engine driven alternators perform very badly on single phase battery chargers with no inductance on the dc side. The conduction peak is only a short interval near the peak of the sine wave, by the time you include selenium rectifier loss, transformer impedance and the alternator reactance you are too far off the peak of the sine wave to get a decent mean current.

First step change the rectifier and I am sure there will be a fair improvement. If you have a variac and you have the potential to get 30% extra voltage from the charger you can try a low resistance inductor in the dc line between the rectifier and battery. This will spread the conduction angle over much more of the sine wave and the alternator reactance will have far less effect. This trick will make things worse unless you can get the extra voltage to change prom peak conduction to mean and you will be looking at something like a 24v output on the ac side of your transformer.

The big virtue of the selenium rectifier as pointed out was that the only test kit needed was your nose. I went to mend lots of TVs as a kid and when I met the small picture and the smell of the rectifier ( worse than cabbage in my opinion) I just went home for a new rectifier.

Flux

[tanner0441]

Flux

I remember those. What was the TV 9 or 12 inch with a big hole in the middle of the chassis with a selenium rectifyer sitting in it, about 2 in square and 6 or 8 in long. You had a shock off the chassis which ever way the plug was wired.

I bet you remember EY51s as well, and anti corona paste.

Off topic so I will shut up...

Brian

[Baling Wire]

I believe that Abyss is correct about the problem.  Charging with 13.8v will not put much current into an almost fully charged battery.  The greater the voltage differential, the greater the charge current.

bw

[bob golding]

think i have found the problem. i changed the rectifier to a silicon one. still no change. then i checked the voltage into the transformer. 219 volts. it should be 230 or 240. checked the output of the generator. 219 volts. the meter on the genny says 240. my multimeter says 219. now i have to find out why the genny is only outting out 219 volts  speeding it up doesnt make any difference. anyone got any ideas? i might be able to get a bit more out of the variac if i rewire it. any ideas on what value inductance i will need flux?

cheers

bob

[dnix71]

219 volts under load isn't bad, but if you have 230/240 no load, then try a smaller load and see what you get.

Your generator may simply not be able to handle the load from the transformer. Speeding up the genny should raise the voltage. I blew an electric chain saw after hurricane Andrew in Miami using an old Onan with a bad governor. I checked the voltage after that and it was over 200 when it should have been 120.

Fixing the governor and slowing down the engine a bit fixed it enough that it could be used to run a fridge.