Voltage drop betwen charger and battery

[Ian_H]

Hi All
We've just had a lithium battery fitted, with a new Victron 30A charger replacing the original 16A mains charger.
I was a bit surprised to see the Victron report outputting 14.2V at 30A but the lithium bluetooth app showing 13.6 at the battery. At 15A the difference was 0.3V.
I will check the voltages with a meter. If confirmed is this voltage drop acceptable/expected?

 

[jwells]

Over what period were you monitoring it?
State of charge of the battery?

 

[Ian_H]

The battery was 90% charged, readings at battery and charger were noted within seconds of each other. Then I changed the max output of the charger and repeated. I assume the voltage drop is over the cable, maybe either due to being too thin or a bad connection ( if the drop is in fact too high).

 

[pappajohn]

If you are using the original wiring then I say that's the problem ....wires too thin.
3% volt drop ((0.43v @14.4v) is acceptable but thin wires will restrict the amps

 

[Richard n Ann]

Next step I would do is measure the charging current, I'm no lithium expert but I would expect to see the current dropping off at 90% charged. Something doesn't sound right

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[treblid]

If confirmed is this voltage drop acceptable/expected?

Normal accepted maximum voltage drop is 3% to 4%. At 14.2V then 4% would be 0.568V so you are on the limit.

It is probably due to the original wiring being retained. In practice, the charging profiles will not be affected as the current will fall away and the voltages at each end of the cable will equalise. The extra voltage drop will lead to a slightly longer charging time.

 

[Ian_H]

If you are using the original wiring then I say that's the problem ....wires too thin.
3% volt drop ((0.43v @14.4v) is acceptable but thin wires will restrict the amps

I suspect the wires didn't get changed, but need to check with the guy tomorrow.

 

[Ian_H]

I guess my biggest concern is safety, could 18W heating over the wire potentially be unsafe? It doesn't sound much to me, but even 5W bulbs get hot!

 

[Richard n Ann]

I guess my biggest concern is safety, could 18W heating over the wire potentially be unsafe? It doesn't sound much to me, but even 5W bulbs get hot!

That's why I suggested measuring the current. Surely it won't be 18w because the current will be much less than 30amps. I presume 30a is just the charger setting?

 

[Ian_H]

That's why I suggested measuring the current. Surely it won't be 18w because the current will be much less than 30amps. I presume 30a is just the charger setting?

No, 30A is the output the victron was reporting at the time. And the battery app was reporting the same current.

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[Richard n Ann]

No, 30A is the output the victron was reporting at the time.

I'll get me coat

 

[treblid]

I guess my biggest concern is safety, could 18W heating over the wire potentially be unsafe? It doesn't sound much to me, but even 5W bulbs get hot!

It will be 18W dissipated in the positive and the negative wires; if they are the same and equal it would 9W per wire with plenty of surface area to dissipate the energy. Not ideal as it's wasted energy but there there will always be some voltage drop in practice. 6% is on the limit.

5W bulbs get hot because the energy is focussed in one small spot.

 

[Ian_H]

OK, so sounds like it's not great, but not really a problem?

 

[treblid]

OK, so sounds like it's not great, but not really a problem?

But not a good job if you paid good money for someone to do the upgrade.

 

[Raul]

That’s a big drop, you don’t want a heating wire, you want a safe conductor. It’s different if the 18w is dissipated by a 10-12m wire vs a 3m wire, to much heat. Also the rate of charge matters, if that’s a 100ah battery, then a 30A charge it’s a 0.3C rate, and the voltage will rise a little, compared to a 0.2C rate.
I would say, definitely undersized cables.

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[Ian_H]

That’s a big drop, you don’t want a heating wire, you want a safe conductor. It’s different if the 18w is dissipated by a 10-12m wire vs a 3m wire, to much heat. Also the rate of charge matters, if that’s a 100ah battery, then a 30A charge it’s a 0.3C rate, and the voltage will rise a little, compared to a 0.2C rate.
I would say, definitely undersized cables.

It can't be more than 2.5m run (1 cab seat to the other) so 5m for positive and negative. The battery is 230Ah. Do these numbers affect your thoughts?

 

[Raul]

It can't be more than 2.5m run (1 cab seat to the other) so 5m for positive and negative. The battery is 230Ah. Do these numbers affect your thoughts?

It does indeed, that’s a low rate for a 30a charge into a 230ah. That tells me the wire is grossly undersized if voltage drops with such a low rate 0.13C

For 2.5m length and 30A at 14v approximately 420w, you want a minimum 16mm2 wire for a 1.6% drop, or, 10mm2 wire for a 2.53% drop. Price for a 5meter cable difference it’s not that great, 10 vs 16.

 

[Ian_H]

Thought I'd post an update on this:
I got a bit more miffed when I looked more closely at the wiring. The new 250W solar added at the same time also runs through the same cables from the distribution board to the lithium battery - potentially another 15A or so at the same time as the 30A mains charger compared to the original 16A max. So I had a moan and the guy agreed to increase the wiring size, although he was still saying it would work OK.
Anyway he added 25mm2 (!) cables from the distribution board to the lithium battery today, and the voltage drop with the mains charger at 30A is now about 0.2V instead of 0.6V, so I think I'm happy with that. Just surprised and disappointed it wasn't done first time round, especially as the company has loads of great feedback on here - which is why I went to them.