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Old 06-23-2012, 11:22 PM   #1 (permalink)
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Lead Acid Science/ Q&A and collabrative brainstorm for solutions

This thread is to ask/answer some scientific questions about lead acid batteries. I think this is a much needed thread to add to this site, as lead acid is one of the most popular chemistries used in EV conversions. The information shared in this thread should be open and available for anyone who wants to access it.

It will also serve as a resource for the online/local EV network that I am starting. This network was inspired by WIKISPEED, which is a network of volunteers working to develop modular efficient gasoline powered cars. The network that I am starting is going to be similar, but focused primarily on EV's and electric vehicle systems. If anyone would like to join or be a part of this please contact me.

Here are some questions that I have about them. If anyone else has questions or answers to common questions, etc please post them.

Do flooded lead acid batteries need vents/fans or can they be kept in an airtight box? I've heard that they produce hydrogen and that this needs to be vented out, which makes sense. On the other hand there are sealed, gel, and AGM batteries that don't seem to need to be vented. Would it be possible to construct an airtight, or vented box to put the batteries in, that is acid resistant, and structurally sound? What sorts of materials could be good for this? What kind of fans do you need for airflow, or do you just need holes, or what?

Would lead acid batteries benefit from a BMS? I know that it is necessary to have one for lithium batteries. If a BMS would extend the life/increase the performance of a lead acid battery pack, then it might be worth adding.

These next questions will deal with Lead Acid battery life/how to extend it.

Typically lead acid batteries last for around 500 charge/discharge cycles. How can we extend this, and how far? To my knowledge lead acid batteries are completely, or almost completely recyclable. If this is the case, what causes the batteries go bad? Is there any way to reverse, or partially reverse this without completely disassembling the batteries, and taking them back to their base components.

What are factors that contribute to the degradation of lead acid batteries? Lets try to look at this both chemically, and practically. Sulfation of the electrodes seems to be one of the main, if not the main factor contributing to lead acid degradation. Sulfation of the electrodes happens from the acid reacting to the lead plates. Sulfation happens most quickly at low states of charge.

Practically, lead acid batteries wear out from being discharged too quickly, being charged too quickly, being discharged too far, being charged too far, sitting at a low state of charge for a long time, and sitting with no trickle charge. Please add any other factors that lead to the degradation of lead acid batteries.

What is the best charging rate for lead acid batteries? I've heard that slower is better, but to what point? It seems that if the battery is charged too slowly it is kept at a lower state of charge for a longer period of time, which I've heard is one of the main causes of degradation. On the other hand, even a small charging voltage applied to the batteries is supposed to completely stop, or vastly slow down sulfation. What rate (preferably in C) is needed to do this?

Now if even a trickle charge can stop the sulfation of the batteries would it be possible to make a small, slightly higher voltage lithium battery to trickle charge the lead acid batteries? For example, even a 100 watt hour battery could supply a trickle charge to the battery pack while it is sitting. This could possibly extend the life of the batteries greatly. In order to maintain this lithium battery, a small solar panel could be added to charge it. It could also be charged simultaneously with the rest of the battery pack during charging.

There are also specialized chargers that are supposed to desulfate batteries. These apply a charge at a specific frequency, which is supposed to knock the sulfation off the electrodes. Would it be possible to have one of these continuously running for a vehicle? It could be powered by a small lithium battery pack and/or a solar panel. If the sulfation is continuously fought, then the battery pack should last much longer, correct?

A scientific look behind quick charging and discharging, shows that if charged too quickly, the battery will charge just the area of the acid right by the electrodes, this is why letting a battery "charged" like this will lose voltage over the next few hours. On the other end of the spectrum is quickly discharging, you may know that a battery that has been discharged too quickly will appear to go dead, and if you let it sit for a few hours it will appear to "gain" voltage.

A clever solution proposed by many, is to use a small capacitor to help level out the load on the batteries.

These are some links that cover this idea more in depth.
The size proposed in the video is not a very large energy storage device. Around 40wh or so. Even at the low energy density of capacitors, this is no more then 10 pounds or so. Also, as it is just 40wh, or 0.04kwh, even at $2000,$10,000 per kwh this is only $80-$400 for a device that could almost completely eliminate the heavy load on a properly sized battery pack. 40 watt hours is the size chosen because it is more than the kinetic energy of an average vehicle at city speeds. This means that even if you drew as much power as your motor could handle to accelerate, or for regen the battery pack would not have to feel any high C charge/discharge rates. It could absorb this load from the capacitor slowly. (side note: possibly use capacitor instead of lithium ion battery for trickle charging/desulfating)

I've heard that flooded lead acid batteries have to be "watered". How is this done, and how often? Would it be possible to make an automated system to maintain the batteries constantly at the proper level?

One of the main ways that I see to extend the life of a battery pack, is to simply make it larger. A battery pack twice the size, should last far more than twice as long. This is because all loads placed on the pack are halved. For example if the max discharge rate on a 10kwh pack is 2C, on a 20kwh pack it is 1C. If on a 10kwh pack the lowest state of charge seen is 40%, then on a 20kwh pack the lowest state of charge seen is 70%.

Now, due to Peukert's Law the more slowly the battery pack is discharged, the more energy it actually has. This means that a battery pack that is twice as big actually has more than twice as much energy. There is a limit to this however. Here is a link to some good info:
Peukert Effect
In the chart you can see for example that at the 1.3hr vs the 0.64hr rate the batteries drawn at approximately half the rate have 130ah vs 119ah, this is a difference of about 9% in capacity. This means a battery pack twice as large will actually store 218% the energy.

Based on this it would seem obvious that the answer to poor lead acid performance is to simply make the pack larger. But of course we run into a few problems with making a larger battery pack. It weighs more, it takes up more space, and it costs more. This means we need a more powerful motor to maintain the same acceleration rate, more room in the car, and more money. However if you look at the cost per kwh levelized cost the larger battery pack should cost less because it should last more than proportionally longer. Now due to increased rolling resistance, a larger battery pack will demand more wh/mi than a smaller battery pack.

*Note* This next part contains the math needed to quantify the difference in range between the battery packs, as well as per battery, etc. This is something that I feel the EV community has been missing for years, and I am happy to contribute to the knowledge pool.

Based on the site's calculator adding an additional 10kwh to a battery pack at 30wh/kg with a coefficient of rolling resistance of just over 0.10 will mean an increase in rolling resistance of about 14wh/mi.

Using some algebra we can determine the level of efficiency needed to have the same miles of range per battery.


This means that unless a vehicle uses less than 155.55wh/mi with a 10kwh battery pack, then the range using a 20kwh battery pack will be more than double the original range due to Peukert's law
Here is a clarification of the math for the specific situation
10,000 wh / 155 (wh/mi) = 21800/ (2x) We use 2x because this gives a range per battery, rather than total range statistic.

we can also use this equation
10,000wh /155 (wh/mi)= 10,900/x
we can then check by subtracting 155 from 168.95 in which case we get 13.95, the difference in wh/mi confirming our math

This means that unless your vehicle is more efficient that 155wh/mi you will actually more than double your range by doubling the size of your battery pack.

This is very important as it goes against conventional knowledge. Supposedly there is a diminishing effect with increasing the size of the battery pack. For chemistries that are affected less by Peukert's law like lithium ion batteries this is true, and it is true for lead acid up to a point. For example making a battery pack 1000 times as big will not give you more than 1000 times the range unless the original battery pack was only a few watt hours. The reason for the increase in range per battery is due to Peukert's law, and looking at graphs you see that it's affect is most prevalent at higher C discharge rates, rates below 1/20 C do not see much effect.

What this means for EV conversions and other lead acid EV's

For vehicles that have a range of 15-40 miles, they are often discharged at a rate of far more than 1C. Increasing the size of the battery pack will more than double your range, and more than double your battery life.

Now this still leaves the factors of reduced acceleration, and less space. Obviously it is not practical to have 2.5 tons of lead acid batteries in every car. The car frame, and motor must be able to handle any increase in battery pack size before changing the size of your battery pack.

New Lead Acid Tech
Lead acid batteries have been around for hundreds of years. There are some newer technologies that interest me. For example bipolar lead acid batteries are supposed to be superior to normal lead acid batteries.

The New Lead Acid Battery | New Energy and Fuel

"So they are smaller, some 40% less volume or 60% the size of a regular lead acid battery.
They weight less too, some 30% less or 70% the mass of regular lead acid batteries."

Bipolar lead acid batteries are supposed to consist of the same materials as regular lead acid batteries, but are supposed to be more efficient due to their layout. They are supposed to be lighter, smaller, more power dense, have longer life cycles, and be able to be discharged farther.

Now due to the fact that they use the same materials, and simpler construction, and have higher energy density, the cost per KWH should be less than half of conventional deep cycle lead acid batteries.

This would provide an alternative which is cheaper than lead acid, nearly as dense as Nickel Metal Hydride, and lasts longer than lead acid. This would be huge for EV conversions and EVs.

If anybody would like to join me in developing some small scale Bipolar lead acid batteries your help is very welcome. I would like to try to build homemade lead acid batteries, both traditional and bipolar and compare the results. If we can develop a modular battery system that can be made by anybody, then the cost of building an EV should be reduced drastically.

Please let me know your thoughts on these topics. The knowledge base here is a great resource, and I'd love it if we could expand it's abilities, and reach.

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