Battery Design

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Nigel Taylor

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Having spent 14 years in battery research and design I thought I should share what I know. However, there are no dedicated places for battery design and so I thought how hard can it be??? I have the model making website and I can just create a battery design website.

It's early days, but I'm hoping this will develop over the next few months into a useful resource.

batterydesign-net-03.jpg

BatteryDesign.net

I know that everybody on here has the ability to literally make anything and hence thought this might be a great resource for you all.

Even a page on the steps in the cell manufacturing process:

Battery Design-cell-manufacture-schematic-02.jpg
 
Battery University has been heavily discredited - reportedly for hoovering up partial and misinformation from the web and promoting it as fact.
I used to refer to it myself.
Sadly, trust once lost may be impossible to regain.

I will follow this with great interest.
 
Battery University has been heavily discredited - reportedly for hoovering up partial and misinformation from the web and promoting it as fact.
I used to refer to it myself.
Sadly, trust once lost may be impossible to regain.

I will follow this with great interest.
Interesting. In a previous life I used that site a fair bit for information. Can you give any examples of misinformation?
 
Not specifically. I'm pretty sure it was a wikipedia entry that i read a few years ago. I haven't looked at it since and can't find it now. There are sparse references to the issue on the web but nothing I can point you to.
 
Anyway, interesting stuff from Nigel. My battery making stalled at stage of building voltaic piles of saliva soaked wadges of tissue paper sandwiched between coins of dissimilar metals. I think my copy of "The boy electrician" has constructional details of lead acid accumulators, but I never got that far.
 
I think my copy of "The boy electrician" has constructional details of lead acid accumulators, but I never got that far.
It's amazing that anyone with an interest in technology survived the middle part of the 20th century. I have the "Hobbies New Annual" edited by F J Camm, wherein you can wind your own mains transforrmer (no earth) and plug it into a light fitting, then, (if you've survived that), guild your own brass items using mercury...
 
It's amazing that anyone with an interest in technology survived the middle part of the 20th century. I have the "Hobbies New Annual" edited by F J Camm, wherein you can wind your own mains transforrmer (no earth) and plug it into a light fitting, then, (if you've survived that), guild your own brass items using mercury...
I'm not sure about the copy I have, but the edition of "The boy electrician" I used to borrow from the library as a child has instructions for making X ray apparatus, amongst other things.
 
Thanks to all for the comments and feedback. If there are specific topics you want to see covered then do let me know. Also, would be great to see some home developed batteries and packs - great way to learn is to build one yourself, best regards, Nigel
 
One topic I would like to know more about if you have time to explain in terms a layman can understand is the reuse of automotive traction batteries for domestic power storage. There was a scheme in Germany and when I heard about it a few years ago there had been 20,000 installations where batteries deteriorated beyond powering cars were repurposed to store domestic power, charged both by solar and off peak electricity and used in some circumstances to send power back to the grid to help in times of high demand (I can only assume this contribution was negligible) As I was told the cells were re-packed into a unit about the size of a washing machine. A PHEV vehicle I worked on a couple of years ago had a battery pack capable of giving 127 Amps but using it at that level would cause significant deterioration, rapid charging and temperature extremes also did it no good. Domestic use by comparison with controlled temperatures, low loads and charging at a reasonable rate to appropriate levels sounds much kinder but I am a mechanical engineer all I know about electrical things is that they run on smoke and if you let that out putting it back is very expensive. I presume if you were to design a battery for domestic use it would be different from an automotive traction battery, how practical is this reuse?
 
One topic I would like to know more about if you have time to explain in terms a layman can understand is the reuse of automotive traction batteries for domestic power storage. There was a scheme in Germany and when I heard about it a few years ago there had been 20,000 installations where batteries deteriorated beyond powering cars were repurposed to store domestic power, charged both by solar and off peak electricity and used in some circumstances to send power back to the grid to help in times of high demand (I can only assume this contribution was negligible) As I was told the cells were re-packed into a unit about the size of a washing machine. A PHEV vehicle I worked on a couple of years ago had a battery pack capable of giving 127 Amps but using it at that level would cause significant deterioration, rapid charging and temperature extremes also did it no good. Domestic use by comparison with controlled temperatures, low loads and charging at a reasonable rate to appropriate levels sounds much kinder but I am a mechanical engineer all I know about electrical things is that they run on smoke and if you let that out putting it back is very expensive. I presume if you were to design a battery for domestic use it would be different from an automotive traction battery, how practical is this reuse?

Hi Ozi, This is an interesting repurposing of batteries. There are a number of factors at play / to be considered. Not sure this will answer everything, but hopefully will help.

As you say, taking a battery from a PHEV and using it in a domestic setting would be sensible in terms of power cycling. The peak and continuous power requirements for the home are quite low. Typically a house in the UK uses ~8kWh of electricity a day (assuming gas heating). Peak loads are probably ~12kW (shower + kettle on together in the morning). Average based on the 8kWh and 24 hours is 0.33kW. Let's assume we use a 12kWh battery pack.

Max discharge is 1C for the house installation, that's quite easy for the old PHEV cells (even at 80% SoH which is the definition used for an aged vehicle pack).

The issues are around the cost of dismantling the original pack and rebuilding it into a new pack for use in a different application. The battery management system will have been set up for use in a vehicle and will expect certain signals/information. You could just use the battery cells/modules and install a new controller. This is ok as long as the new controller can interface with the smaller control boards on the modules. The other issue is the vehicle is likely to have only qualified the cells and controller down to 80% state of health (SoH) and beyond there the cells can behave differently. What I'm saying is the manufacturer of the household packs needs to be talking to the original pack design house to get a number of parameters to ensure that the control system and housing design are safe. All of this is not too bad to do.

Let's assume we have a large solar panel on the roof and we can recharge the panel during the day whilst the house is empty. So, on a normal day we put 8kWh into the pack. There will be losses in the conversion both ways 90% in and out efficiency is a reasonable assumption. For now we can assume the solar panel has more than enough to cover this.

Electricity is ~20p/kWh and so we get £1.60/day worth of free electricity to use at night that we couldn't use from the panel when the sun was shining.

The battery would have probably taken ~3000 complete cycles to drop 20% in state of health (SoH) in the vehicle use. More cycles if it had not been cycled 100%

So, let's assume we can get another 3000 cycles before it reaches 60% SoH. This is the point at which grid storage is assumed to be at it's end of life. Also, at this point our 12kWh pack would have to deliver >100% depth of discharge to give 8kWh. So we would have to curtail the amount of energy delivered to the house. Power delivery should not be an issue as a pack like this should be capable of delivering >5C and hence ~60kW. However, we would have to look at the heat generated in the pack at 12kW and understand how that can work in all ambient conditions. This all assumes the pack is passively cooled and heated (so really will want to operate in a 5°C to 30°C ambient).

In the perfect world this system would save the household £4800. This means we need to get the solar and battery cost below £4800 to make it work financially. If you are charging the pack off the grid and then using the electricity when prices are higher the £/kWh save will be smaller. However, this is not the total story.

Moving forward with electricity prices this equation will get better and this will also give your household electricity security.

Quite a few ramblings here, but hope that helps, there are lots of other reasons for installing this type of system, so please don't take this as gospel, best regards, Nigel
 
The other thing we need to consider is chemistry. The cells used in vehicle applications are high energy density and tend to be higher cost.

Grid applications are not concerned with weight or volume and hence are tending to use Lithium Iron Phosphate cells and these can have a much higher cycle life and are cheaper. This paper on slide 14 shows a cycle life of 12000 to 80% SoH: Degradation of Commercial Lithium-ion Cells Beyond 80% Capacity. (Conference) | OSTI.GOV
 
I had many years ago my grandfathers copy of an old UK book from around the 30's??? with many 'diy' electrical projects- one of which was a battery charger that plugged into a light socket, had a home made diode and used a lightbulb in series (they recommended a 15w from memory for most car batteries)
That was it- that was the entire circuit!!!
A diode and a light bulb in series, connected directly to the mains....
:-O

(in an unpolarised plug that fits into a BC lamp socket either way around...)

OHS would have kittens if they saw that these days lol

It's amazing that anyone with an interest in technology survived the middle part of the 20th century. I have the "Hobbies New Annual" edited by F J Camm, wherein you can wind your own mains transforrmer (no earth) and plug it into a light fitting, then, (if you've survived that), guild your own brass items using mercury...
 
The other thing we need to consider is chemistry. The cells used in vehicle applications are high energy density and tend to be higher cost.

Grid applications are not concerned with weight or volume and hence are tending to use Lithium Iron Phosphate cells and these can have a much higher cycle life and are cheaper. This paper on slide 14 shows a cycle life of 12000 to 80% SoH: Degradation of Commercial Lithium-ion Cells Beyond 80% Capacity. (Conference) | OSTI.GOV
Jeez, that is a very high cycle capacity- I live offgrid here in Australia, running LiFePO4 cells off solar, and I thought mine were good lol- 5000 cycles (13 years) for 80%DOD, and 7000 cycles plus for 70% DOD (near enough 20 years)
(I am expecting mine to outlive me as most of the time they will only be hitting 30% DOD overnight or less)
 
Jeez, that is a very high cycle capacity- I live offgrid here in Australia, running LiFePO4 cells off solar, and I thought mine were good lol- 5000 cycles (13 years) for 80%DOD, and 7000 cycles plus for 70% DOD (near enough 20 years)
(I am expecting mine to outlive me as most of the time they will only be hitting 30% DOD overnight or less)
I assume you see some high ambient temperatures, but is that reflected in the battery temperatures?
 
I am not pushing them hard (atm) with only 1.5kw of solar feeding the 20kwh bank temporarily, but our ambient air can be over 40C for weeks on end in summer- to date I haven't seen them much above 50 while charging, but that could change once the full array is up on the house (when its finished that is lol) then it will have a max of 18kw of solar (three banks, east north and west, 6kw each)- then temp may become an issue...
(I have done a 50kw solar/30kwhstorage install for a 5 cabin B&B that did get uncomfortably high with the same brand in air, so they ended up in an oilbath for cooling- they were hitting 80C which is only 5C off their max, after putting the oilbath in, they were running at 50-60C)
 
Q. for Nigel please.
I'm looking at a solar PV design with battery storage to go in in the next few months. Tentatively 7-8kW peak solar capacity with 10 or 16kWh of battery (about a days average consumption). It's grid tied. Just a case of maxing out the use of the roof so that we have surplus for EV charging, aircon, and some water heating. This is in the UK so ambient temps will be quite low.
1. Are LiFePO4 batteries a good choice (long life and safety I believe) or are there better options for this sort of application ?
2. Is there any non-linearity in the relationship between depth of discharge and the life of the battery packs ?
The batteries have a 10 yr warranty and are specified as 100% DoD, but if (say) halving the DoD would MORE than double the life, that would be useful to know.
Thanks !
 
Heres a thought, could I buy a large battery storage pack, charge it up with economy 7 cheap electricity at night, then sell it back to the grid during peak rates
 

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