Low Voltage Cut Off For Grid Charger

[Dexter-USC]

good day.

I am looking to build and incorporate a low voltage discharge shut off in a grid charger I built for my Hybrid car.(down loaded a posted schematic on hybrid web site) It is a charger that puts out a DC voltage of 177vdc @ .5 amps. its used to balance the nickel-metal-hydrate cells. at this time I will charge the battery pack up as high as it will go and then I use 2 in series 110 incandescent light bulbs to discharge, this has to be monitored closely in order to prevent over discharging.(about 130vdc) then Its a re-charged and discharge cycle 5 or 6 times over a 3 to 4 day time period. is this something you may be able to help me with ?

(I can read and understand basic prints and wiring but will get lost if overwhelmed with to much tech info)

thank you. Dexter-usc

 

[(*steve*)]

Not sure about these nickel-metal-hydrate cells. Are you sure they're not nickel metal hydride (NiMH)?

Is this a Prius?

 

[Dexter-USC]

yes, its NiMH, and its a Honda insight. I do this every 8 to 12 months to keep them working properly.

 

[(*steve*)]

OK. So all you want is to stop the discharge at 130V?

Here is a simple circuit, but it does have some issues I will need to resolve



Before I could suggest a MOSFET and determine the need for a heatsink, I need to know the cold resistance of your lamps.

Please measure the resistance across the bulbs and tell me what wattage they are.

 

[Dexter-USC]

I use two types of bulbs, 2-40 watt or 2-70 watt. when using the 40's I get a smaller bounce back when I remove the load.
I will get those numbers tomorrow morning. would a diagram of what I build be helpful in anyway?

 

[(*steve*)]

The circuit above will have minimal "bounce back" because it can draw a lower current when the voltage is very close to the cutoff.

Sure, a diagram can't hurt.

I'm expecting something like 17 and 30 ohms, but there's a wide margin in the estimates

 

[Dexter-USC]

OK.
here's the ohms reading's for 2-40 watt, 51.0 Ω & for 2-70 watt 29.5 Ω
and a parts list with schematic diagram of the charger.
the 130v cut off is a statement by the diagram author in a later post.

I have modified it by adding a toggle switch between the + & - HV battery pack to transition from charge to monitor voltage and discharge.
I believe its a double poll/ 3 position.

 

[(*steve*)]

I will get back to you soon

 

[(*steve*)]

So the resistance of the 40W bulb can be approximated as 51 + 2.29v ohms (where v is the voltage across the bulb)

For the 70W bulb, the equation is 29.5 + 1.3v ohms.

We might write this down as R = x +yV, where X is 29.5 and y is 1.3.

For n bulbs in series, the resistance would be R = nx + yV

The MOSFET will dissipate very little until the input voltage gets down to 130V.

At 130V (or through a very small voltage range around this) the MOSFET will turn off. The dissipation of the MOSFET will rise and then fall again.

For 2 70W bulbs in series, the resistance is 59 + 1.3v ohms.

So, at 130V, the resistance of 2 70W bulbs in series is about 228 ohms and the current is 0.57A -- sounds reasonable.

More to come...

 

[(*steve*)]

I did the next step numerically by calculating the power in the MOSFET as a function of the voltage across the bulbs

The current through the bulbs is V / (nx + yV), so the power in the MOSFET is (130 - V) * V / (nx + yV)

For either bulb, the max power dissipation in the mosfet is when the voltage across the bulb is 44 volts.

In the case of the pair of 70W bulbs, this is 32.5W.

For the pair of 40W bulbs, the max power is 18.7W

So, assuming we want to use the 70W bulbs, we need a MOSFET capable of 200V, 1A, and 50W dissipation. In practice we'll probably be more conservative. We will also want to heatsink it so it is capable of 40W continuous dissipation (even though the peak dissipation will not last for long)

Next step is to choose a MOSFET.

More to come...

 

[(*steve*)]

I would recommend an FQP6N40C.

In this application you need to consider the SOA of the MOSFET which is operating in the linear region.

I would recommend a 5°C/W heatsink. Beware that unless you use insulating washers and mounting hardware the heatsink will have a dangerous voltage on it near the end of the discharge cycle. I would also recommend a small fan attached to the heatsink.

The risk with this circuit is that the mosfet gets too hot and fails short circuit. This will prevent the lamps from being turned off and the battery will be totally discharged.

I would recommend monitoring the temperature of the heatsink during the discharge. If it stays below 75°C you should be OK.

If you know the approximate time it takes to discharge the battery, it would be good insurance to have a timer disconnect the battery after this time. A failure of the MOSFET will not have such severe consequences if you do this.

 

[Dexter-USC]

the discharge time will vary depending on which cycle its on (1,2 and so on) the newer the cycle the longer the time for discharge. same with charging, the 1st cycle is quicker then cycle 6. I have used a count down timer (such as a AC wall timer) turn and go. its a 2 hr. but if I do not keep a close eye on it, the voltage drops quickly around 130.

the Ohm reading I gave was with both 40 watt combined. with the AC outlet & discharge cord.
I will get individual readings.

 

[Dexter-USC]

I do appreciate your assistance with this very much.