# Programmable Volt Meter (cheap cell balancer?)



## StanSimmons (Sep 3, 2011)

The $38 Programmable Volt Meter from LightObject.com is an interesting little gadget that I heard about from Jack on EVTV. It has two programmable relays built in, and you can set a high/low voltage for each. With a suitable relay and resistor, it will make a dandy little cell balancer (top or bottom balance). 

I'm going to wire it up with a contactor and light bulb and see how it goes for bottom balancing. 

It could also be programmed as a safety backup to kill the AC current to a pack charger when the pack got to a set voltage. 

What do you guys think?


----------



## DIYguy (Sep 18, 2008)

ya, I think it's awesome. I bought one several months ago. The price is incredible. I was looking for this a year ago and couldn't find anything decent. Great little gadget.


----------



## frodus (Apr 12, 2008)

It could be ok as a single cell balancer, but aren't you using it for pack voltage?


----------



## StanSimmons (Sep 3, 2011)

frodus said:


> It could be ok as a single cell balancer, but aren't you using it for pack voltage?


Nope, I'm going to use Jack's design and I'll set the lower limit at 2.60v and the upper limit at 2.77v. The cell will discharge into the light bulb (or resistor) until it reaches 2.60v. The PVM will then disengage the contactor. The cell voltage will creep back up slowly. When it reaches 2.77v, the PVM will again engage the contactor and take the cell down to 2.6v again. This cycle will be repeated until the cell just can't make it to 2.77v. That should be pretty close to 2.75volts.

I could program the other relay for the pack voltage and use it as a safety backup on the charger.


----------



## rwaudio (May 22, 2008)

I've been doing this for awhile now and it does work great (if you have the rest of the parts on hand). I have switched to using a PowerLab6 for discharging since I can use regenerative discharge since I can get some of the energy back and use it again.

I have a few of these meters (both voltage and current) and they work great, and could be used for all sorts of things to monitor/protect your EV.

I also have a pair of meters that are almost identical that run off high voltage AC or DC so if you were monitoring a charger for example you could power it off the same AC for simplicity. Or run it off of pack voltage instead of the 12v battery/dc dc converter.

http://www.ebay.com/itm/350503688293#ht_3127wt_1396

http://www.ebay.com/itm/200445180122#ht_3516wt_1396


----------



## rwaudio (May 22, 2008)

StanSimmons said:


> Nope, I'm going to use Jack's design and I'll set the lower limit at 2.60v and the upper limit at 2.77v. The cell will discharge into the light bulb (or resistor) until it reaches 2.60v. The PVM will then disengage the contactor. The cell voltage will creep back up slowly. When it reaches 2.77v, the PVM will again engage the contactor and take the cell down to 2.6v again. This cycle will be repeated until the cell just can't make it to 2.77v. That should be pretty close to 2.75volts.
> 
> I could program the other relay for the pack voltage and use it as a safety backup on the charger.


A light bulb isn't going to let you draw much current, the resistance is fairly high usually. For a 3.2v cell you need something in the 0.1ohm or lower range.
I was using eight 0.05ohm resistors in a series/parallel arrangement equalling ~0.025ohm to give me a 100A average discharge current. Calculate the desired discharge current using I=V/R IE 3v/0.025ohm = 120A however due to connections and the contactor I was seeing ~110A at 3v.

If you want to go the other way, and have a desired discharge current use R=V/I to calculate the desired resistor to use. make sure you use a resistor of high enough wattage.


----------



## StanSimmons (Sep 3, 2011)

rwaudio said:


> A light bulb isn't going to let you draw much current, the resistance is fairly high usually. For a 3.2v cell you need something in the 0.1ohm or lower range.
> I was using eight 0.05ohm resistors in a series/parallel arrangement equalling ~0.025ohm to give me a 100A average discharge current. Calculate the desired discharge current using I=V/R IE 3v/0.025ohm = 120A however due to connections and the contactor I was seeing ~110A at 3v.
> 
> If you want to go the other way, and have a desired discharge current use R=V/I to calculate the desired resistor to use. make sure you use a resistor of high enough wattage.


Doh! What was I thinking?

I'll go get a .05ohm resistor to give me ~60A draw. That will be 1C for the cells I'm wanting to use.


----------



## rwaudio (May 22, 2008)

StanSimmons said:


> Doh! What was I thinking?
> 
> I'll go get a .05ohm resistor to give me ~60A draw. That will be 1C for the cells I'm wanting to use.


Just make sure it's in the range of 250 watts or higher.

You can also parallel higher resistances to gain power handling while decreasing resistance. For example ten 25 watt 0.5ohm resistors in parallel would be the equivalent of one 0.05ohm 250 watt resistors. A cooling fan blowing over the resistor(s) doesn't hurt either.

I've even heard of people using a steel coat hanger in a bucket of water. (never tried it myself though)


----------



## DIYguy (Sep 18, 2008)

It was RWAudio who put me on to this gizmo so.... Thank You.


----------



## JRue (Jun 1, 2011)

This device really got me thinking about the possibility of using it for both charge protection and low voltage cutoff. 

On the charge side, it'd be easy to set it up as a failsafe to shut off a charger that is going over it's preprogrammed constant voltage limit. Thinking about it more though, what I'd really want in a secondary charging monitor is to either count amp hours or simply time (minutes or seconds) from the point when the charger enters the CV phase. On my battery pack with my Elcon 2500 charging terminates typically around 15 minutes after the charger enters the CV phase. Adding a programmable timer to the voltmeter can give you the option of shutting down the charger if it's taking too much longer than normal.

If both of the voltmeter relays can be programmed for the over limit condition I'd like to set something up like this. For my pack I charge to 124.1 V and like I said it takes about 15 minutes after the pack reaches 124.1 V for the charging to terminate.

Condition 1 (overcharge protection): set relay A to start a timer when the voltage reaches 124.1V. Set the timer to disengage the charger when it reaches (arbitrarily for the sake of just putting a value out there) 25 minutes.
Condition 2 (charger overvoltage protection): set relay B to disengage the charger if the pack voltage exceeds ~128 V.

Don't kill me on the details as this is just conceptual. But if the voltmeter and timer were in one unit this would be a very and cheap charging watchdog.


----------



## rwaudio (May 22, 2008)

JRue said:


> This device really got me thinking about the possibility of using it for both charge protection and low voltage cutoff.
> 
> On the charge side, it'd be easy to set it up as a failsafe to shut off a charger that is going over it's preprogrammed constant voltage limit. Thinking about it more though, what I'd really want in a secondary charging monitor is to either count amp hours or simply time (minutes or seconds) from the point when the charger enters the CV phase. On my battery pack with my Elcon 2500 charging terminates typically around 15 minutes after the charger enters the CV phase. Adding a programmable timer to the voltmeter can give you the option of shutting down the charger if it's taking too much longer than normal.
> 
> ...


A set up like that should work just fine. The timer may be redundant but since redundancy is the goal here it wouldn't hurt as long as there are no false alarms that leave your car "uncharged". I will be using the AC powered programmable volt meter as a fail safe that will turn off the charger if the pack voltage goes above a certain value. 

These are handy meters that have all sorts of uses. I'm also using one to charge the battery that is powering my PowerLab 6, I have a 50Ah 12v pack of A123's so that I can use the regenerative discharge function on the cell I'm testing, but I need to keep the battery in a middle SOC. Not so full that I can't regen current back into it, but high enough that the charger doesn't throttle back due to low voltage. So I turn on the charger below 12v and turn it off at 13.6v So regardless of the portion of the charge/discharge cycle I'm on the charger is happy and will just keep working. I might set the charger to do some automated long term cycle testing of a cell so this would work perfectly. The correct voltage/current from a trickle charger or power supply might do this as well but I already know this works.


----------



## frodus (Apr 12, 2008)

Stan,

Can you set the voltmeter to latch off so it never comes back on?

I have an application where if the voltage drops below a setpoint, the relay drops out and doesn't come back on.


----------



## StanSimmons (Sep 3, 2011)

frodus said:


> Stan,
> 
> Can you set the voltmeter to latch off so it never comes back on?
> 
> I have an application where if the voltage drops below a setpoint, the relay drops out and doesn't come back on.


I think so... I won't be able to test it until next week. Here is a link to the manual: http://www.lightobject.info/download/file.php?id=55


----------



## frodus (Apr 12, 2008)

Nope, doesn't look like it does. It appears to self reset. I need it not to reset. 

Basically I need it for a LVC detect for battery discharging to disconnect the load.


----------



## rwaudio (May 22, 2008)

frodus said:


> Nope, doesn't look like it does. It appears to self reset. I need it not to reset.
> 
> Basically I need it for a LVC detect for battery discharging to disconnect the load.


It all depends on your settings.
There is the HV and LV setting

(and where the meter is being powered from)

If you power the meter from the load side of 12v and with the correct HV LV settings, it could use set to disconnect all loads from 12v as well as itself.

To reset you would need a momentary push button that powered up the meter only. Then if the voltage is in the correct range the relay within the meter would turn on (turning on your contactor etc) and restore 12v power.

The down side is you power the meter all the time.


----------



## PhantomPholly (Aug 20, 2008)

I'm a bit puzzled by this thread, which isn't surprising since electrons are not my specialty.

Questions:

When charging, won't the charger raise the voltage higher than the cell?
If #1 is true, won't the charger be shut off prematurely (e.g. once you shut it off, the actual battery voltage is lower)?
When discharging, doesn't the load lower the voltage (sag)?
If #3 is true, might not the load be shut off prematurely (e.g. once you eliminate the load, the battery voltage is still within limits)?
Other thoughts occur to me that might be useful for protecting banks of cells in EVs. Since this device has relays, could you configure it to act as a "bank bypass" if the voltage being contributed by a particular bank of cells dropped too low? So, rather than shutting off the entire pack as it gets low, just shut off the bank of cells most likely to be damaged by the drain?

It seems like there would be "companion products" to this device, measuring resistance and current - but I'm not sure how they would be useful.


----------



## m38mike (Dec 27, 2008)

frodus said:


> Nope, doesn't look like it does. It appears to self reset. I need it not to reset.
> 
> Basically I need it for a LVC detect for battery discharging to disconnect the load.


Why don't you have it trip a 2-relay cut-off. The relay's stay tripped until the power to them is turned off and turned back on. I can post the diagram of how to wire them if this would work for you. 

I use the 2-relay trick on an rpm meter to shut down power to my main contactor in the event of a runaway motor (i.e. it exceeds redline).


----------



## frodus (Apr 12, 2008)

m38mike said:


> Why don't you have it trip a 2-relay cut-off. The relay's stay tripped until the power to them is turned off and turned back on. I can post the diagram of how to wire them if this would work for you.
> 
> I use the 2-relay trick on an rpm meter to shut down power to my main contactor in the event of a runaway motor (i.e. it exceeds redline).


Essentially that's what I've decided. I thought about it a little and remembered how to wire self latching relays. One of the EV200 contactors I bought has Aux contacts. I'll wire it so the contactor self-latches. Any interrupt on the Volt-Meter (light object) internal relay would temporarily break the contactor drive circuit and the contactor would drop out. I don't even need a second relay, just the one inside the Volt-Meter and the contacts on the contactor.

Thanks for the tip!


----------



## StanSimmons (Sep 3, 2011)

I did a write up on my blog about building a bottom balancer with this meter for under $100. Included are detail build and programming instructions.

http://st1100ev.blogspot.com/2012/09/budget-bottom-balancing.html

I would appreciate any critiques and comments from you guys.


----------



## Ziggythewiz (May 16, 2010)

oops, replied to old thread


----------

