ITT: job exam/application (possibly related to ohms law)

Poyndexter

Alright, I'm applying for a job as a UPS tech (involves battery backup systems for commercial properties). I must pass an exam before I can move on to the next stage of the hiring process. The lady from H.R. gave me a study guide for said exam, which she said covered all aspects of the test.

My problem isn't that I don't understand any of it, it's that I don't understand one single part of it. Google hasn't really provided me with the help I was looking for (your google-fu may be stronger than mine). So my question is this:

I'm asked to "Be able to calculate total resistance, voltage, total power, current,float voltage"

I have 2000 hours electrical experience under my belt and went to an electricians school. I have a pretty good grasp of Ohms Law, but until today had never heard of float voltage. I understand from reading on line that the float voltage should be high enough to maintain the full charge of the battery, but not so high that it will cause excessive gassing.

How the hell do I calculate the float voltage for a given cell/battery?

RoundBoy

off the top of my head, if the study guide does not provide definitive answers for this, then you can ignore it.

I have NO experience in electrical matters such as this, but from what i can gleen, 'float voltage' is the required voltage to maintain the battery's charge while not overcharging it. Its the thing that most good chargers do. I really only see that term in Lead-acid batteries (cars / UPS equipment (!) )

While I have yet to find a single calculation for it, it does relate to time.

Interesting reading :

If you connect a lead acid battery to a power supply that's set to deliver the same voltage as the battery can hold when it's fully charged, you'll charge the battery, and you won't overcharge it. This is called a "float" charge; it's not the fastest way to get amp-hours into the battery, but you can leave the charger connected forever without toasting the battery.

Unfortunately, a float charge will never quite top off a battery. And if you leave a lead acid battery on float forever, it'll slowly sulfate, just as if it were left flat briefly. A "topping charge" of about 2.4 volts per cell (14.4 volts, for a 12 volt battery) is needed about every six months, for maximum battery life.

"Float" of Stand-By Service: Hold battery across constant voltage source of 2.25 to 2.30 volts per cell continuously. When held at this voltage, the battery will seek its own current level and maintain itself in a fully charged condition.

http://www.batterywholesale.com/charging_tf.html#float
http://www.batterywholesale.com/batinfo.html
Control voltage : 6.8 to 6.9V /6V battery 13.6 to 13.8V /12V battery Float charging compensates for load fluctuations.

All said and done, the float voltage is the current required to maintain the battery at full charge .. any higher Voltage in will charge the battery . I would assume that given a proper diagram of the circuit in question with battery voltage & resistances supplied, you could calculate the voltage required to keep the battery stable via Ohms law.