sla in parallel with lifepo4

[mechanix]

hello all
I was wondering if its possible to 3-12v sla batteries (36v) in parallel with a 36v lifepo4 battery to get longer range. I'm kicking around the idea of attaching my old sla's for those extra long trips. I know that lifepo4's sit at a slightly higher voltage and was hoping this wouldn't pose a problem.

joel

 

[Malcolm]

Hi Joel

I considered this a while ago myself and posted the following information on another forum a few months back. I've since decided that life (pun intended) is complicated enough and now plan to stick with lithium, so I didn't take this any further. You might find it useful anyway.


I’ve been thinking about this type of arrangement for a while for a lightweight car conversion, mainly to keep costs down. I don’t like the idea of weighing down a vehicle with a load of lead, but at the same time can’t afford a decent-sized lithium ion pack, so I’m considering a compromise until the price of lithium cells comes within reach. I’d appreciate any feedback on possible pitfalls of this approach. The idea is to build a small 108V pack as follows. Maximum discharge current would be around 300A.

Nine 12V Hawker Genesis lead acid 42AH batteries (ex backup power supply: £180 from eBay). Weight: 135 kg

108 x 3V LifeBatt LiFePO4 10 Ah cells (at around £20 per cell = £2160). Weight: 41 kg

The LifeBatt cells would be arranged in parallel groups of three cells (so 30 Ah), which would then be wired in series to give a nominal 108V.

The idea is to connect the lead acid string in parallel with the lithium ion string to give a nominal 108V 72 Ah pack. Because of the high discharge rate I would only expect to get about 25 Ah from the lead acid batteries, so the total available capacity would be roughly 50 Ah at 108V, or 5.4 kWh. At an estimated energy consumption of 250 Wh/mile this would give around 20 miles range.

The attraction of using this combination of cells/batteries is that their voltages match up neatly. The voltage of four LiFePO4 cells closely matches the voltage of a lead acid 12V monoblock (made up of six 2V cells). In addition, lead acid and lithium ion both use a similar constant current/constant voltage (CCCV) charging characteristic.

To test how the cells work together I’ve done a few measurements on a 36V 10Ah LifeBatt pack connected in parallel with a 36V 42 Ah Hawker Genesis pack. The parallel packs were charged from roughly 80% state of charge using a Curtis 36V/30A high-frequency battery charger adjusted to give a finishing voltage of 43.8V with the current tapering to about 4A. This is not the optimum charging characteristic for the lead acid batteries, but my main priority was to charge the more expensive lithium ion cells correctly. Here are some figures for comparison:

Final charging voltage:
Pb 43.8V (14.6V per battery)
Li 43.8V (3.65V per cell)

Voltage fresh off charger (after disconnecting the two strings):
Pb 40V (13.33V per battery)
Li 42.0V (3.5V per cell)

Resting voltage after 8 hours (strings disconnected immediately after charging)
Pb 38.5V (12.83V per battery)
Li 39.8V (3.32V per cell)

After measuring the resting voltages above I connected the lead and lithium strings back in parallel just to see how they got on together. Because the lithium pack has a slightly higher voltage it naturally feeds the lead pack. The initial current from the lithium string was about two amps. This tapered off quite quickly, reaching 0.12A after one hour, and 0.07A after three hours, so it seems that the strings could be left connected in parallel after charging without any real problem.

According to the Hawker spec sheet, at 20% state of charge (the lowest I would want to go), the voltage of the lead acid battery drops to just over 11.8V, which corresponds to 2.95V for the lithium cells. This seems a little high as a finishing voltage for lithium, but I guess it’s better to be on the safe side.

So far so good. Next I’d like to do a discharge test with the two strings connected in parallel, but I need to find a suitable load first. I’ve still got more questions than answers: How will the two strings share the load? What would be the optimum balance between lead and lithium string capacities? What effect will the greater sag of the lead acid batteries have on current sharing? Frank Wykes said in his brief article that “in no circumstances should high and low power sets of cells be operated in parallel, even if the voltage is the same”. Does this apply to LifeBatt and lead acid, and why

EDIT: I should of course mention that mucking about with batteries like this could shorten their lives and, in the case of LifeBatt cells, using a non-approved charger will invalidate your guarantee.

 

[dogman dan]

If it's a hassle to figure out, just run one pack, then switch to the other. If the battery pack plug is easily accesible, it would take like, 5 seconds to do the switch. That way you can tailor the cutoff point to the chemistry, and charge seperate, etc.

 

[mechanix]

thanks for the replys, I also was just thinking about installing a on-off-on toggle switch to switch between the batteries, but thought that maybe the lower drain rate on the sla's in parallel would allow me to get more ah out of them during the trip. what kind of range can I expect with just the ping 36v in conjunction with a bl36 on a 26" rim with the stock controller and nominal peddling?

joel

 

[dogman dan]

I'm not quite sure, I run the bd 36, and in warm weather, get 20 miles on a ping 36v-20 ah. Lately though, that is down about 25% from the cold weather. With the bl36 , I would expect you should get around 20 miles minimum, even in near freezing weather. 25 miles or more in warm, and that is at full throttle.
That is a good point, about lowering the amp rate of the drain on both packs. I'm not sure how you would be able to track how drained the sla's got easily. I would be a bit worried about draining them too far down when paraleled. I'm really an electronic simpleton, so I kiss everything if possible.

 

[safe]

I'm not sure how you would be able to track how drained the sla's got easily. I would be a bit worried about draining them too far down when paraleled.

I've tried series and parallel for my NiCads/SLA.

The advantage of parallel is that you get a more constant voltage across the entire ride. Parallel reduces SLA "sag".

The advantage of series is that I can connect the SLA's so that two 12 volt cells are connected in parallel before it gets passed in series to the rest of the circuit. The parallel SLA's within the series circuit means that I get a low tech form of balancing. At the beginning of the ride I get lots of voltage (power) and it "sags" linearly towards the end more like standard SLA's. It's easier on the SLA's.

The Winner?

I've decided to go series because I like the low tech balancing that it achieves. If I had a balancing circuit (for the SLA's) I would probably switch to parallel because you get a more consistent ride. In parallel the SLA's are very vulnerable to getting a damaged "runt" cell. The NiCads are in parallel either way so they always self balance.

 

[dogman dan]

Asuming that the lifepo4 was a 20 ah, and the sla's 12, he might run the sla's dead without knowing it, since the lifepo4 would keep the voltage up. But really I have no clue if it would work like that, or not. Would the lifepo4 want to charge the sla once it's voltage dropped? I know it wouldn't in series, but would it in paralell?

 

[Malcolm]

Would the lifepo4 want to charge the sla once it's voltage dropped? I know it wouldn't in series, but would it in paralell?

That's the way I imagined it would work. To get a decent cycle life for the lead acid you would want to stop discharge before the state of charge dropped below say 20%, which for Odyssey batteries is just below 2.0V per cell open circuit voltage.

In the case of LiFe cells, once you've reached an open circuit voltage of 3.0V per cell there's not much capacity left in them. This works out quite neatly at four lithium cells equals six lead acid cells. On a bigger pack you could adjust the relative cut-off voltages of the lead acid and lithium packs by adding or removing a lithium cell, although that would make charging more complicated

It's obviously difficult to measure OCV in real life use since the cells can sag a lot below that under load. The shapes of the discharge curves are also different, so I'm not sure how they would behave dynamically together. I'd like to try it out some time just out of curiosity.

 

[vanilla ice]

You should get a lot more watt hours out of your lead if you parallel them rather than run them alone then switch to the 2nd life pack. Prolly get twice as much out the lead assuming an extremely high C situation..

Safe's comparison isn't really the same kind of comparison to switching packs out like what you're sayin- cause he's always got the same amount of Wh hooked up either of the two ways he's wired them up. With the switch you're running half the Wh then swapping over the other half.

 

[safe]

The whole point of running the SLA's in parallel is to cut the "C" rate. So it makes no sense to run them separately.

But the other issue is about balancing of the two paths (SLA and "Other") and what happens when the SLA's start to drain in capacity.

I've found that the SLA's do indeed "balance" with the "Other" chemistry, but that balancing simply adjusts the rate that the two paths drain. If one chemistry is more energetic than the SLA's (like NiCads, NiMh, or LiFePO4) they will prevent the "sag" by participating more in the delivery of energy.

The "problem" is that when the SLA's finally do start to run low (as the overall system runs low) the individual cells revert back to their normal tendency to become imbalanced. The "runt" cell is still there no matter if you ran pure SLA's or with a parallel assist.

The moral of the story is:

"Putting SLA's in parallel with another chemistry does NOT eliminate 'runts'."

 

[dogman dan]

It's proven that I don't know jack about circuits and such, but In my mind it seems that to protect the sla's you would want a lvc on them so they wouldn't drain past the voltage of your choice. The problem in this case, as I see it, is the total watthours avaliable in the sla pack is much smaller than in the lifepo4 pack, so wouldn't the sla's run out first, while you never noticed since the lifepo4 continues to provide ample volts? If the sla pack was big and the lifepo4 pack small, you could just run till you felt the difference, when the lifepo4 lvc cuts out. Obviously I am guessing here, but wouldn't it work best if the avaliable watthours were similar in each pack?

Of course, with some experience with the setup, you could just keep the sla's ok, simply by using an odometer and being conservative. You could stop every mile or so and check the sla pack with a voltmeter to get some Idea how they are doing at various points on a ride that you do regularly.

 

[Anonymous]

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[safe]

Hyperion LBA10 Net Balancer

The LBA10 Net Balancer is used to get the maximum performance from your Li-Po or A123 battery packs in a voltage range of 2S-6S in one of three ways:

"Stand-Alone" discharge (without a charger or discharger attached)

Loaded Discharge (with a load device for discharge load)

Balance-While-Charging (using a LiPo or A123 compatible charger)

http://holmeshobbies.com/product.php?productid=88&cat=7&page=1

$39.95

--------------------

First I wonder what is meant by "Stand Alone" verses "Loaded Discharge" and what is different. Does "Stand Alone" mean that the balancer works when the batteries sit doing nothing and then "Loaded Discharge" means that it works only while riding the bike? Probably.

I don't care about charge balancing for my SLA's, so the fact that this is designed for Lipos during charging is unimportant to me. All I would want is a way to protect my SLA's with a runtime balancer.

The price is right... $40... it actually might make economic sense to do this to protect the SLA's from the cronic problem of "runt" cells... :wink: (it would extend the life of the "runt" a great deal I suspect)

SLA's in series (combined with other chemistries or not) will gradually create imbalances and "runts" will die off early. This could be the cure. Anyone know much about these? Has anyone ever tried to use them just for their runtime capabilities and not their charging capabilities?

The battery voltage range is claimed to be: 26.0V (2S-6S LiPo or A123) so the idea of connecting 48 volts worth of SLA would probably require addition costs and then the whole thing goes downhill. (I think I asked about this a little while ago elsewhere and I had concluded it wouldn't work with just one)

On this thread:

http://endless-sphere.com/forums/viewtopic.php?f=14&t=4332&start=30

...someone had suggested something costing $235. So starting at $40 is at least making some progress. :lol:

Maybe this product could be combined with something like a resistor on each cell at the point that the balancer connects so that it would be able to handle the higher voltage?

Okay... I'm suggesting a "hack"... but a $40 solution for SLA's is worth thinking about.

 

[safe]

Anyone?

As a "hack" is there any way to use a $40 balancer designed for something else as an SLA balancer?

 

[vanilla ice]

At the cell level? You can be the first! Seems like it'd be a huge pain tho eh.

 

[safe]

Balancers are still too expensive. I'm just fishing for ways to balance my SLA's for around $40 and this "hack" could do it.

I suspect that since this balancer is looking for a range that is at most 26 volts that my four cell 48 volt SLA set would need to use two of them. So I'm already up to $80.

At present I've figured out a "caveman balancer" that exploits the parallel connection of the SLA's and it seems to be working, but I'd like to drop the NiCads and go back to just the SLA's one day. The SLA's without a balancer create "runts" too fast and it gets expensive to replace the "runt" all the time.

--------------------

As for the main topic:

"Yes" we can conclude that mixing chemistries works great. You can change the behavior of the battery by altering the way you mix the chemistries in both series and parallel configurations.

SLA + LiFePO4 or NiCad ----> Parallel : Eliminates SLA "Sag".

SLA + LiFePO4 or NiCad ----> Series : SLA's placed in parallel can reduce the "C" rate and give low tech balancing. SLA "Sag" will be present.