Lifepo4 pack producing AC voltage

[GoneDiving]

Has anyone ever had Lifepo4s producing an AC voltage? I have 48 280ah Eve cells connected in 16S3P and the whole pack is showing 119.5 vac and individual cells are showing ~6.5 vac when totally isolated from other equipment. No BMS is fitted. Batteries SOC is 70% and solar/ charging is disconnected.

Ideas on what is causing this and how to stop it? The AC spikes are tripping sensitive navigation equipment on a boat.

Thanks

 

[cafesport]

The times I've seen this the culprit was a bad charger. Never anything near what you have described. 16s3p cells (16*3.3v) should measure 50 plus volts and 6.5 VAC/ cell doesn't add up either. We need to hear the rest of the story. What was it doing before you built this project and why no BMS. Inquiring minds want to know.

 

[CharlieJ]

OP: "Has anyone ever had Lifepo4s producing an AC voltage?"
To answer that question directly, no, because it is absolutely impossible to occur. That said, @cafesport is spot on so pull that thread.
The effects of a component of a charging source that is acting improperly or has failed may be observed at the battery terminals, but the batteries did not produce it.

 

[Bmarler]

I have seen alternators produce ac voltage when a diode fails.

 

[Iggy]

Shut you charger down, does the problem go away? Use another meter to verify your readings.

 

[Portage_Bay]

OP said

solar/ charging is disconnected.

If by "/charging" he means his battery charger then it can't be the source of the problem. He says the whole pack is showing 119.5 VAC. I'd try killing any AC source, shore power, inverter, anything. If that doesn't work open every AC breaker and switch. Unplug anything that runs on AC. Still got AC on the DC circuits? Then totally disconnect the bank and look again. Still got AC? Suspect the meter.

 

[twistedtree]

So the battery cells are completely isolated, i.e. nothing connected to either terminal, no BMS electronics, and you measure 6.5VAC across the terminals? I think you just have a metering problem. Is the meter set to AC manually, or is it automatically selecting AC? If so, manually set the meter to DC.

I think you are just picking up a stray reading. When in AC meter mode, can you also measure the frequency? That might give you a hint where it's coming from. Your meter leads are an antenna to some degree, for some range of frequencies.

Tell us more about the nav equipment that is getting tripped? If it's imposed AC, it's coming from somewhere else. What other equipment is running at the time, especially inverters and engine driven alternators? Is the equipment that's tripping AC or DC. Both alternators and inverters will create DC ripple. It's usually inconsequential, but occasionally equipment is susceptible to it.

 

[SteveK]

It cannot be a coincidence that the AC voltage is around what is expected from AC supply. Stop swimming around that boat until solved.

 

[GoneDiving]

Thanks everyone for your replies

*See this for the full details on how the pack was built.

An idiot's guide to building a big lithium bank

Found ya. (-; So how do you rate the slipway and trades ? It's one we have on our list for our 60 ft, 65 tonne and 7.5ft draft Generally very good. I'll expand via PM. 60ft and 65t wouldn't be a problem. For scale, I'm 54ft and the vessel behind is 60. Check your messages

www.trawlerforum.com

*The fault was initially found with everything connected. Individual components were then isolated one by one until the isolated pack was measured at 53v DC and 119 vac then an individual cell was measured at 3.3v DC and 6.5vac. Again this is with the pack and cells isolated ie nothing connected to the pos or neg terminals.

*No alternators, inverters or other chargers were connected at the final measuring.

*Meters were set to manually read vac. Frequency not measured yet

*Reading were taken using three different meters, two of which were professional quality units.

*A new cluster of instruments is being fitted: 2 finder plotters, radios, radar and auto pilot. The fault was found when the auto pilot would turn on but we could not enter the settings menu. It's a TMQ AP4. When connected to another battery, the autopilot operates normally. No other new electronics were connected for fear of damaging them.

Thanks again, every one.

 

[twistedtree]

That is some serious space-alien stuff going on. I'd measure frequency next in hopes of liking it to some source.

Based on your location, I assume the AC part of your power system (when connected and operational) is 230V/50hz?

I can see why you are perplexed. It makes no sense at all.

 

[Delta Riverat]

I agree. Batteries do not make AC. The only thing I can think of to produce this kind of behavior would be a self clearing internal short or open which could produce pulsed DC but not a true reverse polarity AC.

 

[GoneDiving]

That is some serious space-alien stuff going on. I'd measure frequency next in hopes of liking it to some source.

Based on your location, I assume the AC part of your power system (when connected and operational) is 230V/50hz?

I can see why you are perplexed. It makes no sense at all.

Correct on the 230/50hz

I don't have the capacity to measure hz. I'll get the boffins to bing the appropriate meters.

Yep, that's why I'm doing a lot of head scratching. Makes no sense for an isolated cell.

Thx

 

[GoneDiving]

I agree. Batteries do not make AC. The only thing I can think of to produce this kind of behavior would be a self clearing internal short or open which could produce pulsed DC but not a true reverse polarity AC.

Agree. Also strange that the vac is approximately double the dc voltage. Almost as if it's building up in a cap or similar then discharging.

Anyway, waaay above my pay grade so am having some adults have a look.

Thx

 

[READY]

Possible it is induced energy if the DC cables are close to an AC cable? It has been a lot of years since I got my degree in physics but I cannot think of any other way to get an AC reading from a battery "IF" it is truly isolated. As others said my first suspect would be from a charger or inverter but if that is disconnected???

 

[TowLou]

May consider to look at you neutral and ground on the AC side. Something else may be bonded that should not be.

 

[READY]

Inquiring minds want to know...What did the adults find to be the source of the AC?

 

[Insequent]

Mike, you have a steel boat if I recall coorectly and aswell as norml 230VAC at 50 Hz, also some 3 phase AC on board. Now a faraday cage will block external electromagnetic fileds, but what if there is a ource of electromagnetic fields inside the cage? Did you have a genny running or shore power connected etc?