Diy constant current Generator / battery charger

Ruurd

Hallo, i have build a small constant current Generator / battery charger. I have a small 2 stroke suzuki gs750 generator (220v 50hertz and constant rating 550 va) and a 220v/42v 750va transformer. i have put in series a 31,5 uf cap (220v side) and on the 42v side a fbr and connected it directly to my 24v 180ah battery bank.
This gives me an nice constant 15 amp charging current.
Left unattended this wold overcharge my battery's. I also got a Victron BMV 702, it has a programmable relais. (https://www.victronenergy.nl/battery-monitors/bmv-700#manual)

Now is the question witch settings i have to use to stop the charging ?

( I'm Dutch and English isn't my best, sorry for that)

Estragon

The short answer is it depends... on the battery you are charging (check with manufacturer for specs) and, assuming lead acid battery, the temperature (cold battery=higher voltage, hot battery=lower voltage).

Ruurd

Yes, the are maintenance-free lead acid semi traction 12v 180ah battery's , don't no the manufacturer out of my head, but can check tomorrow. i can program te relais for battery SOC and for max. battery voltage. i am thinking to go for SOC 95% and let the solar do the rest of it. I tried to connect the 42 dc output to the mppt controller, but the mppt controller and gen don't work good together. The generator starts to "hunt" a lot.

Estragon

Sounds like some sort of AGM or gel battery, which aren't at all happy with overcharging. I wouldn't use a SOC to control. The only way to calibrate SOC is with regular specific gravity measurements, which you can't do on a sealed battery.

Best bet would be to get spec for recommended absorb voltage and end at that. You will need this info to set up the solar controller properly anyway.

Is the battery location likely to get much above room temperature? MPPTcontroller should have temp compensation but doesn't sound like the generator system will.

jonr

I think you would be much better off buying a properly designed battery charger.

westbranch

Once you go past 90% with most MPPT chargers you are into the phase called ABSORB and this is where you have constant voltage and diminishing Amperage...  best to leave this phase and the FLOAT to the Solar controller. 90 to 100% ....

Marc Kurth

Charge your batteries using a constant voltage with an upper limit on current - not a constant current. This is how normal battery chargers work.

When the batteries are being fed a constant voltage, the current draw will eventually fall as they approach full charge. The batteries are fully charged when they are drawing very low current. The battery manufacture will give you the proper charging voltage and ending amps.

Holding a constant current means that you will increase the voltage higher and higher to hold the constant current as the battery approaches full charge. This should only be done under special circumstances as part of a recovery process.

Marc

Ruurd

Today i got the generator working with the mppt controller, and have no more problems with charging the battery bank. The generator supplies a +- 400 watt base load en the solar panels give the rest. it works very nice. When the panels give more power the generator throttles back. I set the max. charging amps at 22 amp. 18 amp voor the battery's and 4 amps for the load. Absorb voltage at 28.8v and float at 26.4v. During the test today i got 18-19 amps going to the battery bank, witch is good voor a 180 ah battery bank. I also sent a email to the battery manufacture for the battery spec's but so far the haven't answerd yet.