Battery Pack Voltage != Cell Voltages

Good Evening All-

I just finished construction of my first 3x CREE XR-E light. I'm using 3x 1050ma current regulators from DX- they share common positive /negative power supply and the output positive is shared between all three LEDs.

I'm using 4x NiMH cells in series to power the unit.

When I measure the voltage across each cell, I get the following:

1.15
1.15
1.11
1.01

When I measure the pack voltage I get
3.79

It's late, I'm really tired... so what am I missing?

(Current draw on the battery pack is about 3.0A)

Thanks much-

Jason

purduephotog said:

When I measure the voltage across each cell, I get the following:

1.15
1.15
1.11
1.01

When I measure the pack voltage I get
3.79

Click to expand...

That's really impossible. Are you measuring the voltage under the same conditions in each case? For example when you measure the cells individually and when you measure the pack voltage, is the load (or absence of load) the same each time? Are you measuring the pack voltage at the battery terminals, or at the input to the regulators?

It's not impossible, especially not under load. It's probably resistance in the battery holder or the wires from it that drop the voltage...

shadowjk said:

It's not impossible, especially not under load. It's probably resistance in the battery holder or the wires from it that drop the voltage...

Click to expand...

Granted. I was sort of assuming the batteries were connected nose-to-tail, but of course they could be mounted side by side in a holder with springs and other connections between them that would add resistance. That would definitely lead to the observed effect.

When there is a current draw of 3 amps or more it becomes really important to have solid low resistance connections in the battery holder and other parts of the circuit. Ideally good tight connections with no steel springs in the current path.

Mr Happy said:

That's really impossible. Are you measuring the voltage under the same conditions in each case? For example when you measure the cells individually and when you measure the pack voltage, is the load (or absence of load) the same each time? Are you measuring the pack voltage at the battery terminals, or at the input to the regulators?

Click to expand...

Everything was measured under load- the pack has the cell positions open so that I can insert the VMM leads. I measured across each cell, then measured the pack output across the output leads.

Here's todays numbers (different cells)
Under load:
1.19
1.18
1.09
1.16
Pack terminals
4.01

Regulator input
3.86

The pack itself is a Radioshack with multi springs and wires- and the output is tiny little leads from a 9V snap connection. Looks like I'm losing ALOT of voltage across those leads.

The measured voltage to two of the CREEs is 3.27 and 3.32 (I can't access the third CREE easily)

I am using the Tenergy LSD cells I've referenced in a different post- getting ready to discharge test some that have been held idle for a couple of months.

Thanks again- I'll look at the wires as the main problem.

Jason

purduephotog said:

The pack itself is a Radioshack with multi springs and wires

Click to expand...

If I may say so, this is your problem. Those plastic battery holders are suitable for currents much less than one amp (mainly for electronic devices that draw a few milliamps). They are really not suitable for high power loads as there is too much resistance in the steel springs and stampings that make up the battery contacts.

In fact, at higher currents in the several amp range there is a risk of thermal damage from the springs overheating.

I would suggest locating or constructing a high power battery holder using copper or brass contacts and with no coiled springs. Alternatively you could make resistance mods to your existing holder by using lengths of copper braid to bypass the steel bits. (At high currents steel is more of a resistor than a conductor.)