Incrementally adding DC batteries

[svetz]

Overall the resistance doesn't change much as SoC changes, but I found this article that seems to match observations:

... ohmic resistance ... increases gradually in discharge with the decreasing SOC.

Probably not enough to worry about, I saw about a 10F rise from ~30% to 15% SoC at .5C; before that it was pretty flat. It would be interesting to hear what anyone else has seen (@robaroni?).

 

[offgriddle]

Currently, I have two, parallel 12 volt lifepo batteries with built in BMS's but the manufacturer says do not use that model if I wish to connect the batteries in series to make 24 volts, not sure why yet, maybe it's limitation of the BMS's. ~ Also, it was my understanding that when combining a new lead acid battery with an old one, the old battery doesn't equalize, thus, essentially improve in performance, instead, the new battery degrades in performance overtime as a result of making up for the lack of performance of the old battery.

 

[Craig]

Currently, I have two, parallel 12 volt lifepo batteries with built in BMS's but the manufacturer says do not use that model if I wish to connect the batteries in series to make 24 volts, not sure why yet, maybe it's limitation of the BMS's. ~ Also, it was my understanding that when combining a new lead acid battery with an old one, the old battery doesn't equalize, thus, essentially improve in performance, instead, the new battery degrades in performance overtime as a result of making up for the lack of performance of the old battery.

Something I cant wrap my head around is if the batteries are in series and you throw energy into them does all the energy go through the first battery to get to the next one. So with a built in BMS could you overpower it by putting batteries in series?

 

[SCClockDr]

This may help you get a handle. How to correctly interconnect multiple batteries to form one larger bank.

Edit: Fixed link.

 

[svetz]

This may help you get a handle. How to correctly interconnect multiple batteries to form one larger bank.

Just an FYI the link isn't working.

 

[SCClockDr]

Just an FYI the link isn't working.

Thanks svetz! It's fixed.

 

[b.james]

Something I cant wrap my head around is if the batteries are in series and you throw energy into them does all the energy go through the first battery to get to the next one. So with a built in BMS could you overpower it by putting batteries in series?

Mate in series what you do to the first cell or battery you do to them all.
If charging at 20 amps then 20 amps flows through the whole string .

Normal BMS's should not be affected. Maybe some dropins have BT or communication boards that dont work well with a series attached friend but they should be made that way or you don't buy them.

 

[offgriddle]

Something I cant wrap my head around is if the batteries are in series and you throw energy into them does all the energy go through the first battery to get to the next one. So with a built in BMS could you overpower it by putting batteries in series?

Good question, because each battery is connected in series then essentially the electrical current would flow into the first battery in the series, flowing through and out of the first battery and into the second battery and so on. Does this mean anything in particular? Well in my humble opinion, if all of the batteries in the series are in the same state of electrical, "health", and specification, then it should not amount to a hill of beans. At least, I have never heard of any advantage and practice or recommendation regarding doing anything like occasionally swapping the batteries around in a series arrangement.

 

[Craig]

Mate in series what you do to the first cell or battery you do to them all.
If charging at 20 amps then 20 amps flows through the whole string .

Normal BMS's should not be affected. Maybe some dropins have BT or communication boards that dont work well with a series attached friend but they should be made that way or you don't buy them.

Thanks this is what I figured but i just didn't know the physics of it.

 

[svetz]

Just when I thought I was getting it.... In this video Jack says the cycle life (aging) isn't based on the C-Rate or [high] temperature as much as the cutoff or max charging voltage. It's from 2013 so I'm not sure if it matches current thought as I thought I'd heard Will say differently. The paper I cited previously was from 2012 so even older.

So based on this, for that first year to minimize IR you might want to set your charge parameters to cutout at 80% and low-voltage cutoff at 20%. Hedge your bets and do both?

At about 35 minutes in he talks about LiFePO4 chemistry. Thanks to Ulmo who posted this originally, interesting information!

 

[offgriddle]

Just when I thought I was getting it.... In this video Jack says the cycle life (aging) isn't based on the C-Rate or [high] temperature as much as the cutoff or max charging voltage. It's from 2013 so I'm not sure if it matches current thought as I thought I'd heard Will say differently. The paper I cited previously was from 2012 so even older.

So based on this, for that first year to minimize IR you might want to set your charge parameters to cutout at 80% and low-voltage cutoff at 20%. Hedge your bets and do both?

At about 35 minutes in he talks about LiFePO4 chemistry. Thanks to Ulmo who posted this originally, interesting information!

I'm currently operating my lifepo's with 80/20 methodology, sure there are times when I want a bit more run time but it's okay as ultimately I prefer not to charge to maximize and discharge to minimum.

 

[Will Prowse]

Here's a fun article: https://www.google.com/url?sa=t&sou...FjAUegQIAhAB&usg=AOvVaw094jKAx06i96hYzK8-ojqt

2.4. Battery Degradation and Second Life Application
Lithium batteries for EVs are subject to two mechanisms that shorten life-time and deteriorate
performance, namely cycling capacity loss and calendar capacity loss. The former depends on the
number of battery charging/discharging cycles, while the latter depends on the state of charge, aging
time, and expose of the battery to high temperatures. Specifically, cycling capacity loss is typically
attributed to the formation of solid electrolyte interphase (SEI) layer, structural changes in the electrodes
and loss of lithium during battery charging/discharging. Calendar capacity loss is attributed to battery
self-discharge and side reactions that occur during the energy storage period [29].
It is estimated that LFP batteries can support at least 2000–2500 cycles in electro-mobility
applications, for example, daily use of a charge and discharge cycle for seven years, until the remaining
capacity reduces to 80% of the initial battery capacity. This allows its use for another 1000–2000 cycles
until the capacity reduces to 60% of the initial capacity. When this occurs, the aging process of the
battery has advanced to point that the voltage drop does not allow further use of the battery [30].

After the end of the useful life of a battery for electro-mobility purposes, typical applications include
its use as energy storage unit in smartgrids or uninterruptible power supply. The main characteristic
of these applications is the lower stress that the battery cells suffer, enhancing thus the durability of the
battery pack. In the context of this work, the primary use phase considers a 24 kWh LFP battery with
efficiency of 80% used for 2500 days in an EV, while the second life application considers the case of
using an LFP battery as an energy storage unit in a smart building for 1500 days (or equivalently, four
years), taking into account the average home consumption in Spain in 2010. In the scenarios that refer
to the use of the same battery in the primary and secondary application, it is further assumed that the
efficiency drops from 80% to 75% due to the aging of the battery.

 

[gnubie]

I think bigclive on youtube has abused lifepo4 cells without any ill effects, but he also drove a nail through a lipo and couldn't get it to catch fire either, so not sure if that proves anything or not!

 

[Will Prowse]

Here's another one on how LiFePO4 creates and dissipates heat: https://www.google.com/url?sa=t&sou...Vaw2bv4q_ADox8x7hiiPPvswi&cshid=1570605818583

The thermal stability of cathode materials plays a key role in lithium-ion battery safety, and the
electrochemical-calorimetric measurements show good performance in studying the thermo-
electrochemical behaviors of LiFePO4.
(1) The lower rate, the greater charge-discharge capacity, the flatter charge-discharge platform,
and the smaller voltage difference between the charge platform and discharge platform. Therefore, the
LiFePO4 battery had a smaller polarization and a better reversibility at low rate. With the increasing of
the rate structure of cathode material LiFePO4 was partially destroyed, electrochemical performance
deteriorated.
(2) The whole charge-discharge stage of LiFePO4 battery accompanied exothermic process
and endothermic process at low charge-discharge rate (0.1C, 0.2C). When the rate increased to 1.0C,
only exothermic phenomena existed. With the rate increasing heat production rate and the enthalpy
change during the charge-discharge process of LiFePO4 battery increased.
Based on the experiment results obtained in this article, we could conclude that appropriate
charge-discharge rate for battery should be choose in order to avoid safety problem, which initiated by
internal heat accumulation of battery.

 

[Will Prowse]

Oh another thing that we forgot to discuss! Matching of cells. The closer they are matched, the better. I read another study showing the difference of mismatched system over a long duration. Horrible results. I'll try to find that study. Matching cells is very very important.

 

[svetz]

Saw an interesting idea by Nef003 about using a device between old and new battery banks to regulate them to avoid the different IR issue.

I suspect it's not done due to costs (I was thinking a >500 amp 12V variable resistor to match the current in the banks); but that seems wasteful... seems like altering the voltage would do the same thing more efficiently... perhaps some sort of buck/boost converter? Any ideas on how it might be practical?

 

[eddie1261]

I have a Ruixu 100ah. On the positive side is a 50 amp bolt on line fuse. To that is connected the cables the run to the charge controller and 12v power block. At some point I will add a second battery exactly like the one I already have to get 200ah. Other than the jumpers from + to + and - to - do I need to do anything else? I don't need another fuse on that second battery or anything, do I? I can't see why I would, but I figured I'd ask.

 

[brianlowe]

Is all this balancing and differential aging relevant to all chemistries, or just LiFePO4 ?

I'm about to commit to 4 x 12v 100Ah Gel Lead Acid batteries (arranged 2s2p to give 200Ah at 24v) at a third of the price of the equivalent in LiFePO4. The project may get a whole rethink after 2 years, so I don't need 2000 cycles right now, and these promise 600 cycles. In 2 years time I'll know better whether to trade up to LiFePO4 (or whatever IBM come up with (see this blog about a new chemistry)) or just replace or supplement what's there.

Am I being silly?

 

[Craig]

Balancing is an issue but not as big of a deal because you only have 2 to deal with not 4 I would arrange in 2p2s though because the parallel batteries become one and will match volts if you do 2s2p they will all have a different voltage. In your cost calculation have you figured that 100ah gel batteries only have 50Ah usable.

 

[SCClockDr]

Is all this balancing and differential aging relevant to all chemistries, or just LiFePO4 ?

I'm about to commit to 4 x 12v 100Ah Gel Lead Acid batteries (arranged 2s2p to give 200Ah at 24v) at a third of the price of the equivalent in LiFePO4. The project may get a whole rethink after 2 years, so I don't need 2000 cycles right now, and these promise 600 cycles. In 2 years time I'll know better whether to trade up to LiFePO4 (or whatever IBM come up with (see this blog about a new chemistry)) or just replace or supplement what's there.

Am I being silly?

I would amend your usable capacity to 100AH @24 volts. The accepted SOC operating range for all lead acid chemistry batteries is 50% > 100%. The hardest part of the charge cycle to complete is the 90-100% portion due to the batteries internal resistance exhibited above 90% charge.