Battery monitor shunt, Lithium.

[Neeves]

I am wiring up a battery monitor, like the Xantrex of 20 years ago, but this one is designed for application with a Lithium battery pack. A cautionary note is that the shunt gets warm (or hot?). For the meter I input total capacity when the battery is full (or empty) and the devices records amps in and out and calculates capacity at any given time. It also displays voltage, a little image of battery capacity and records historic data - simple and I think common stuff. AFAIK the unit is like the NASA Lithium unit - and other similar devices.

Geem has mentioned that the B2B charger, I think its the B2B charger, also get warm and he has found that cooling the unit with a simple computer fan increases efficiency.

As I'm wiring up, or will be soon, should I incorporate a fan, for the shunt. Its sensible or easier, to do it now and or place shunt near the B2B unit. I have a couple of standard computer fans, about 100mm square and one big one of about 120mm square (I've already incorporated another 120mm square fan as extra cooling for the refridgeration but the fridge and power devices are not near each other).

Jonathan

 

[Fr J Hackett]

Does the Peukert effect apply to Lithium batteries? My Link monitor allowed you to apply a factor that supposedly gave a more accurate indication of current use, the system was AGM batteries.

 

[Baggywrinkle]

One thing that is absolutely essential: DO NOT USE SS WASHERS BETWEEN THE LEADS AND THE SHUNT!!!!

This will definitely heat up the connections and cause your shunt to appear hot, maybe even melt the plastic bits.

On shunts ....

Most leisure battery shunts are designed to need no active cooling, but it is always a good idea to keep everything in a well ventilated area with a reasonable clearance around it for heat dissipation.

The shunt should have an ampere rating and is very low resistance, so the shunt itself should not get very hot at normal charging or load currents - double check it is not operating near its max. current rating, especially when charging lithium or if you are running a big inverter.

The attached electronics that measure the voltage difference across the shunt to derive current might get warm, but neither should need active cooling.

If you want to active cool, then cool the compartment as a whole IMO.

 

[B27]

What's the resistance of the shunt?
How much time will it spend with a high current?

I have a 100A shunt here, it's 75mV at full scale, that's 7.5W
You can get Hall Effect sensors to reduce the heating, but at big currents even fat cable has resistance.
As do fuses.
A few watts is enough to raise temperature over time.
Don't forget that the ft cables can also conduct heat from say an inverter to a shunt.

 

[Poey50]

Does the Peukert effect apply to Lithium batteries? My Link monitor allowed you to apply a factor that supposedly gave a more accurate indication of current use, the system was AGM batteries.

Peukert is close to 1 for LFP. People use figures between 1.01 to 1.03 to dial into their monitor if it's sophisticated enough and they are fussy!

 

[noelex]

A shunt should not need active cooling. If you mount it in a hot location just make sure that the current capacity of the shunt comfortably exceeds the maximum expected current. As shunts with up to 1000A capacity are readily available, this should not be an issue.

Other devices such as DC to DC chargers and MPPT controllers are more of a concern. It is not unusual to see installations where the manufacturers’ recommendations regarding mounting locations to provide adequate air flow are difficult to follow. Even when these requirements are met the derated performance at higher temperatures can be a nuisance and active cooling can be worthwhile.

 

[jdc]

My 'shunt' (what a terrible term, a hangover from the meter's perspective - it's a resistor put in series with the battery!) has a resistance of 0.1 mOhm.

Thus at 200A it's only dissipating I² x R = 4W.

At normal currents (say 10A) it's only dissipating 0.01W. And it's a big bit of metal, so I'v e never known it to get the slightest bit warm.

 

[Neeves]

Our shunt is 100 amp, max of 150 amp (the alternative was 350 amp which seems a bit excessive). The shunt is brass (a yellow metal with matching brass washers and spring washer). The shunt itself is three, parallel, heavy duty, silver coloured, wires. I have not measured them but they could be 3mm OD and maybe 40mm long. The whole lot is housed in an acrylic base. The negative of the shunt will be connected to a heavy duty brass (more yellow metal) bus bar with each ancillary cable to its own pole. The positive from the battery simply goes to positive circuits - no contact, see next sentence, with the shunt. There is a very low duty wire, 0.3mm, from the positive terminal of the battery to a separate terminal on the shunt. There is only one negative cable on the battery pole, it goes direct to the shunt and the cable will be a max of 200mm long, the cable from the shunt to the bus bar will be 50mm - 100mm. There are 5 negative cables on the bus bar with poles for more.

I'm not sure but all the poles seem to be stainless, including the battery. They are all 'stainless' coloured.

The eye connectors on the ends of the cables are all, sort of, mattish grey colour, standard AFAIK eye connectors. The eye connectors will interface directly with the bus bar, no washers,

The shunt and bus bar are located with all the other electrical kit, inverter, B2B, circuit breakers, isolator switch, water meter etc., and eventually this coulomb meter The shunt, the bus bar and the battery terminals are in full view when you open the door - I'm wondering if the battery and the bus bar should have covers ....... ? or I think they should but what? ..... acrylic boxes as covers.....?

There is no ventilation, the only 'holes' are where cables enter or exit - I'm thinking of a fan, my big computer fan.

But a fan, which by all accounts will be welcomed by the B2B and inverter , is no use if it simply stirs up warm air - I need to think how to introduce fresh air - which will be difficult in an Australian summer (maybe move to somewhere geographically colder . ).

I'd post a photo - but I have not taken any yet.

I have wondered why it is called a shunt.

Thank you all for your comments.

Jonathan

 

[noelex]

Our shunt is 100 amp,

100A continuous is a small shunt for a marine lithium system. What is the application? Is this a marine system, an RV system or something else?

 

[William_H]

Shunt is so called because in traditional moving coil meter which measures current the "shunt" effectively bypasses most of the current to increase the measurement range of the meter. In modern digital current measuring you are effectively measuring a volt drop across the shunt. So current shunting is not such an obvious analogy.
It is vital when wiring a shunt that the measuring wires are connected to the shunt resistance in such a way that they measure the volt drop across the resistive element. So that any resistance in the main power wire connections do not contribute to the resistance where current is measured. Of course there should be very low resistance in the bolt on connections but not always. ie not connected via main power lugs.
I imagine in Neeves set up the resistance of the shunt is a useful series resistance to limit max current when charging. If it gets hot then it is doing just that. However as said a shunt in usually made of solid metal easily able to dissipate heat generated to the air and via the cabling. Perhaps a heat sink could be useful to attach to the main current bolts. A piece of aluminium sheet painted black or even a proper made ali heat sink as used in cooling transistors could work to allay concerns. Or just make the power cables attached a rediculously large size to carry heat away. ol'will

 

[KompetentKrew]

One thing that is absolutely essential: DO NOT USE SS WASHERS BETWEEN THE LEADS AND THE SHUNT!!!!

This will definitely heat up the connections and cause your shunt to appear hot, maybe even melt the plastic bits.

…

Thanks very much for that, now I have to scour my whole electrical system to see how many times I've screed up.

 

[Neeves]

Thanks very much for that, now I have to scour my whole electrical system to see how many times I've screed up.

I found a stack of heavy duty cables attached to both the positive and negative poles of the battery. I took the negative pole apart - it was a bolt threaded into the battery. The bolt was extra long to take 4 eye connectors. I took 3 connectors off and found the bolt had had to be long to accomodate 4 eyes and was too long for just one. I put an overlarge nut in to pack it out - then decided - poor practice or not I'd live with 4 eyes until I could find the right size bolt M8, short enough, or cut one down.

After all - if US, UK Oz etc etc can live with 5 eyes I can live with 4.

100A continuous is a small shunt for a marine lithium system. What is the application? Is this a marine system, an RV system or something else?

We are quite used to living within a 100 amp restriction - it worked on our catamaran - though it was not lithium. We survived with lead 400amp/hr bank, of which 200amp/hr could be used, we don't see the need for a 'larger' bank and are working with a 200amp/hr Lithium battery.


Serious question:

What on earth do RV owners need for larger than 200 amp/hr lithium batteries. Where do they keep' the stuff'.

Jonathan

 

[Neeves]

Shunt is so called because in traditional moving coil meter which measures current the "shunt" effectively bypasses most of the current to increase the measurement range of the meter. In modern digital current measuring you are effectively measuring a volt drop across the shunt. So current shunting is not such an obvious analogy.
It is vital when wiring a shunt that the measuring wires are connected to the shunt resistance in such a way that they measure the volt drop across the resistive element. So that any resistance in the main power wire connections do not contribute to the resistance where current is measured. Of course there should be very low resistance in the bolt on connections but not always. ie not connected via main power lugs.
I imagine in Neeves set up the resistance of the shunt is a useful series resistance to limit max current when charging. If it gets hot then it is doing just that. However as said a shunt in usually made of solid metal easily able to dissipate heat generated to the air and via the cabling. Perhaps a heat sink could be useful to attach to the main current bolts. A piece of aluminium sheet painted black or even a proper made ali heat sink as used in cooling transistors could work to allay concerns. Or just make the power cables attached a rediculously large size to carry heat away. ol'will

This explains why there is a 0.3mm wire as a sensor or probe (I've seen both terms used). I have not yet connected the sensor (nor the meter) - I'll be making up the sensor tomorrow.

Thanks Wil,

Jonathan

 

[noelex]

Serious question:

What on earth do RV owners need for larger than 200 amp/hr lithium batteries. Where do they keep' the stuff'.

Jonathan

One of the beauties of lithium batteries is the potential to use high draw appliances even with a relatively small battery bank

Like boats, many RVs are enjoying the benefits of running high draw AC appliances. Some are switching over, at least partially, to electric cooking. Especially in hot climates, such as Australia, electric cooking reduces the heat build up inside the RV and solar works efficiently to replace the energy consumed.

This is viable with only a small lithium bank, at least if you avoid low capacity BMS units. Look at what Kelpie or Pete 7 are doing on their yachts. This same versatility applies to RV living, although there are some important differences.

I am still not sure of the application in this case (RV, camper trailer ?) and these details matter when designing a system. Not everyone has the same priorities so being unable to run high draw appliances may not be a concern, but it is important to be aware of the restriction imposed when selecting equipment. Shunts are not expensive so I would make sure this is not a bottleneck with a lower maximum current delivery than other components in the system.

 

[Baggywrinkle]

Serious question:

What on earth do RV owners need for larger than 200 amp/hr lithium batteries. Where do they keep' the stuff'.

Jonathan

... and they have all that roof space for permanent solar panels. They are a bit more akin to MoBos than Sailing boats, as to move anywhere they need the engine, and usually also have a beefed up alternator.

 

[B27]

The down side of using a bigger shunt is that resolution of small currents will be affected.
Depending on what you want to achieve, you could consider measuring e.g. the inverter current separately.
There are lots of sensors from the automotive sector which go in the +ve side, or there are hall sensors which are isolated.

 

[Neeves]

There seems to be a perception with the focus on the all electric 'unit', yacht, RV etc. when considering Lithium. Yes - people got that route but its not for everyone.

Most people owning a yacht and RV already have a perfectly good stove, and often oven, they comes as standard and last 'for ever'. Our house in Sydney was built in the 1960s, we replaced the hob and oven at the end of the 1990s and the new hob and oven look good for another 20 years. Our hob and oven on Josepheline should last at least another 25 years (making 50 in total). We do have an air fryer at home but its life expectancy is low - they are made cheaply.

It is difficult to replicate cooking with gas using electric - as many Chinese restaurants will attest.

You can instal lithium as a replacement for lead without the need to discard the hob and oven.

I suspect gas will be with us, the RV, sailing community, for some years to come. We find a 9kg cylinder lasts, for ever, and its not an inconvenience to re-fill it. We use gas to heat water, for showers.

Of RVs - there is a move in many locations to curse the wild camping RVs - they are disliked by the locals who find them a blight on the landscape and many Councils restrict their ability to wild camp (how can you call it wild camping in an all singing and dancing RV?). The move is to point the RV in the direction of a registered caravan park - where they plug into mains water, electricity and dump facilities - negating the need for a large Lithium bank. If you don't believe this, it may be different in Europe, just rent an RV in NZ. Too hot to cook in a caravan or RV on gas - use the air conditioned camp kitchen - common place in Australia (though a bit busy during school holidays ).

To replace our lead battery bank, 400amp/hrs on Josepheline, which still has many years of life in it could be achieved by replacing with a 200amp/hr Lithium (and a lot of extra kit) - and we could have saved another 40kg. Yes we could have extended the Lithium bank to 400a/h, discarded the hob and oven - but why? Installed an electric hot water system - but why. And with another yacht with an existing hob and stove - discard them as well.

And going all electric is meant to protect the environment, hob and oven to land fill etc etc.

It works in a new custom built yacht, or RV - but that is the route of the few, not the many.

100A continuous is a small shunt for a marine lithium system. What is the application? Is this a marine system, an RV system or something else?

It is not essential to consider 100amp continuous for many people even with Lithium. Its not 'something else' its still main stream to use gas and it is possible to replace a large lead battery bank with Lithium without the need to go all electric. No...you are not using the Lithium to its full potential - but that's accepted for yachts. Many buy yachts capable of high latitude sailing - how many actually do it? Most yachts over, say 35', can be sailed round the world - many are simply used as floating caravans.

Jonathan

 

[Neeves]

The down side of using a bigger shunt is that resolution of small currents will be affected.
Depending on what you want to achieve, you could consider measuring e.g. the inverter current separately.
There are lots of sensors from the automotive sector which go in the +ve side, or there are hall sensors which are isolated.

The instruction for installation of our shunt and meter is to instal a 0.3mm copper wire from the positive pole on the battery to a specific terminal on the shunt. The distance is maybe 0.5m. I was surprised that the wire was not supplied.

But you imply I can go to the automotive or electronic retail sector and buy what is required off the shelf....?? I know I can buy 0.3mm copper wire off the shelf, I googled it, but a 'device' (sensor, probe) made up seems a better option considering that all the other cabling is so robust. I have been conjuring how to crimp an eye big enough for the positive pole such that it is secure with 0.3mm wire


Jonathan

 

[noelex]

Of RVs - there is a move in many locations to curse the wild camping RVs - they are disliked by the locals who find them a blight on the landscape and many Councils restrict their ability to wild camp (how can you call it wild camping in an all singing and dancing RV?). The move is to point the RV in the direction of a registered caravan park - where they plug into mains water, electricity and dump facilities - negating the need for a large Lithium bank.

An RV and yacht have different requirements and goals.

An RV in Australia is very unlikely to travel to different countries so the frustration of coping with different propane fittings is eliminated. In addition, the RV can drive up to a propane source and simply swap a bottle, much easier than the same job in a yacht.

Still it is a shame in an RV to install lithium batteries and not be able to use high draw convenience items, but as always this is the owner’s decision.

 

[Neeves]

Shunts are not expensive so I would make sure this is not a bottleneck with a lower maximum current delivery than other components in the system.

I'd have put it another way:

Shunts, brand name, high capacity shunts - are expensive - especially if you buy one of 350amp when you only need 100amp.

AFAIK the specification for a shunt for marine application and terrestrial application are exactly the same - I am open to education and if someone can tell me that RVs have a unique specification I'm all ears - I know nothing about RVs.

However we went to a camping show, about a year ago, as camping and sailing overlap for their demands for refrigeration etc and I was quite amazed by the presence of exhibitors flogging Lithium for both caravans and RVs. But I dislike towing a big caravan, tried it, and buying and driving a mobile home is not our scene.

Jonathan