Is Voltage sag damaging to batteries?

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David LaFerney   10 mW

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Is Voltage sag damaging to batteries?

Post by David LaFerney » Nov 14 2016 11:01am

I recently added voltage meters to the bikes that I put together for my wife and I...

Image

After a few rides with them, it's clear that whenever you "get on it" the voltage sags around 3-4 volts - probably more at times. So if you consider 3.3 volts per cell to be the danger zone (42.9 for my 13s packs) does that include sag? If so, then do we need to start pedaling and stop gassing at around 46-47 volts to preserve the battery life?

It's not really a big deal even if that is the case - we are riding mostly for exercise, and the motors are more to help make it more fun than to eliminate pedaling - no more long uphill grinds - and no more worry about going too far out and struggling to get back (we aren't kids anymore.) On our typical 10 - 14 mile ride we are only using maybe 2 volts out of an initial 53.3v charge. I've started only charging to 4 volts per cell so that they are at least down to storage voltage when we finish without having to discharge.

If anyone is interested - the volt meters are "waterproof" items off of eBay ($4.24) I made the housings/mounts out of kydex. Very nice addition if you don't already have an actual on-board voltmeter in my opinion.
Last edited by David LaFerney on Nov 14 2016 11:16am, edited 1 time in total.

miuan   10 kW

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Re: Is Voltage sag damaging to batteries?

Post by miuan » Nov 14 2016 11:14am

Absolutely depends on the type of cells you use. But my rule of thumb is, above 10 per cent sag, you are really hammering your batts.
Best to stay within 5% sag for the most part. In your case, that would be 2.5 volts off pack voltage. Then again, quality 18650s will handle more abuse than cheap lipo pouches.

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Re: Is Voltage sag damaging to batteries?

Post by David LaFerney » Nov 14 2016 11:22am

5-10% of resting voltage? Right?

They are supposed to be good cells - Panasonic 18650 3400 mah. If I were doing it over I would configure the packs as 12s5p instead of 13s4p - sacrifice the performance that we aren't using anyway to gain battery resiliency. That would have taken 8 more cells, but would still fit in the frame triangle I think (although it's tight on the ladies bike.) Next time... For now we are just going to go with it, and if it's hard on them it just is I guess. Although I could use the software to govern the maximum wattage... I might try that - right now it's about 880 watts more or less and ungoverned, but I can turn it down to 750. Or 500, or 250, but I don't want to go nuts about it. What's a little bit of battery life anyway?

Without a voltmeter on the bike you would never even know.

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Ykick   100 GW

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Re: Is Voltage sag damaging to batteries?

Post by Ykick » Nov 14 2016 11:50am

Damage from sag really depends on the cell temperature. Short durations of sag on cool or barely warm cells isn’t really damaging IMO.

I did some freezer tests on RC Lipo cells years ago and witnessed some crazy voltage sag (down to 2-2.5V/cell under 1-2C load) all without any detectable long term damage. Some of those test cells I continue to use to this day.

Those tests were conducted on sub freezing cells and I believe the reason for the lack of damage is that the cell temperatures never climbed into critical territory.

A quick burst here and there, probably wouldn’t cause any practical or noticeable cell degradation. But riding around towards end of charge, internal cell temps already elevated? Hammering them long and hard will most likely cause some damage.

How much? Nobody really knows…
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Re: Is Voltage sag damaging to batteries?

Post by Sunder » Nov 14 2016 11:36pm

Ykick wrote:Damage from sag really depends on the cell temperature. Short durations of sag on cool or barely warm cells isn’t really damaging IMO.
^^ This. It's not the sag itself that causes the damage, but the temperature.

All batteries have an internal resistance. The higher the resistance, the higher the sag. The resistance is "real" in that it's not just the voltage dropping, the energy is actually lost. How? Into heat of course. It's that heat that damages the pack.

So, the voltage under sag, doesn't damage batteries the way that the low voltage at rest does (Which is purely chemical). It damages it by heat.
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dogman dan   100 GW

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Re: Is Voltage sag damaging to batteries?

Post by dogman dan » Nov 15 2016 7:09am

What they said.

Your sag is typical, and should not result in enough heat to damage the cells.

Stop riding when you reach your target stopping point voltage, at rest. You can just coast a few seconds to check that out. Slow down a little bit when you see your target voltage under load. Best to lower the discharge rate when the resistance of the cells increases at the very end of the discharge.

You do want to avoid double digit sag for sure. When sag gets that bad, the battery will get hotter, and hotter always worse.

A larger pack will sag less, ride much more perky, and theoretically last longer. The lower the c rate you hit your cells with the better.

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Re: Is Voltage sag damaging to batteries?

Post by 4LivesPerGallon » Nov 15 2016 2:06pm

David LaFerney wrote:I recently added voltage meters to the bikes that I put together for my wife and I...

Image

After a few rides with them, it's clear that whenever you "get on it" the voltage sags around 3-4 volts - probably more at times. So if you consider 3.3 volts per cell to be the danger zone (42.9 for my 13s packs) does that include sag? If so, then do we need to start pedaling and stop gassing at around 46-47 volts to preserve the battery life?

It's not really a big deal even if that is the case - we are riding mostly for exercise, and the motors are more to help make it more fun than to eliminate pedaling - no more long uphill grinds - and no more worry about going too far out and struggling to get back (we aren't kids anymore.) On our typical 10 - 14 mile ride we are only using maybe 2 volts out of an initial 53.3v charge. I've started only charging to 4 volts per cell so that they are at least down to storage voltage when we finish without having to discharge.

If anyone is interested - the volt meters are "waterproof" items off of eBay ($4.24) I made the housings/mounts out of kydex. Very nice addition if you don't already have an actual on-board voltmeter in my opinion.

With Lithium battery packs: A voltage sag like yours indicates the pack is too small.

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Re: Is Voltage sag damaging to batteries?

Post by David LaFerney » Nov 15 2016 11:29pm

With Lithium battery packs: A voltage sag like yours indicates the pack is too small.

I'm sure that's true to some extent. Bigger is always better in some way I suppose. But then again if it does what you want for a reasonable life span (hopefully) and at a price that you can afford...

Your sag is typical, and should not result in enough heat to damage the cells.

Stop riding when you reach your target stopping point voltage, at rest. You can just coast a few seconds to check that out. Slow down a little bit when you see your target voltage under load. Best to lower the discharge rate when the resistance of the cells increases at the very end of the discharge.

You do want to avoid double digit sag for sure. When sag gets that bad, the battery will get hotter, and hotter always worse.

A larger pack will sag less, ride much more perky, and theoretically last longer. The lower the c rate you hit your cells with the better.


That's reassuring - If I don't flog it too hard it sounds like it will be alright. Thanks.

Again, If I were doing it over I would configure the battery for less peak voltage, and more reserve amps. Just speculating, but if that resulted in less voltage sag - and assuming that voltage sag negatively affects performance - that might not really result in much of a performance loss on average. ?? It would surely help the pack last longer.

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dogman dan   100 GW

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Re: Is Voltage sag damaging to batteries?

Post by dogman dan » Nov 16 2016 9:23am

True enough, for some 4v sag under load is not acceptable.

I'm making some assumptions here, as usual. I assumed the bike is not a hot bike, that it has around a 20 amps controller. 4 amps sag under load with just a 20 amps load is not great. But it is typical, and batteries I have used that sagged that much still lasted many years.

It would be better of course, to have less sag. I often put 30 ah of battery on my bike, just so I can enjoy nearly zero sag. Bike rides much better, perkier, doesn't slow much till I've ridden over 30 miles. lots nicer. But I am carrying 25 pounds of battery. This works for me because the bike is built specifically to carry a lot of battery.

Most bikes not built to carry more than 10-15 pounds of battery, and 30 ah is very costly, so you live with a little sag.

There are of course, small and relatively light batteries that pack a lot more punch, and sag very little when as small as 10 ah. I have some of those too. But they can burn your house down

It's all a lot of tradeoffs. For the vast majority, a 10 ah battery made with moderately good cells is all they need. and they live with at least 4 v sag under load. Their battery still lasts 3 years, maybe 4.

Cells keep getting better, but because of cost, the standard size will remain about 10 ah. People flip out enough about the cost of the battery enough as it is. So the typical size is not going to increase to 15 ah.

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Re: Is Voltage sag damaging to batteries?

Post by miuan » Nov 16 2016 11:02am

dogman dan wrote:Cells keep getting better, but because of cost, the standard size will remain about 10 ah.
Actually, in many instances it's mere size/mass of the battery that keeps us from running more capacity. Dogman's bike is an exception to the rule :)
I could afford many times more battery than I am using, but I refuse to bother with the added weight when all I need is 4-6kg to get around town under any circumstances.
Same with high end production ebikes, they already cost thousands euro as they are, and a few more cells would add just a tiny fraction to that price, but most bikes benefit from less battery and weight.

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Re: Is Voltage sag damaging to batteries?

Post by MrDude_1 » Nov 16 2016 1:34pm

in my case, I have never been constrained to stay within any OEM packaging and I am designing it to go fast, and last... so a 20ah battery IS a small battery to me.

Its not the cheapest route, but I also put a 60+a draw on my battery and see almost no voltage drop.
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Re: Is Voltage sag damaging to batteries?

Post by Mündawg » Nov 17 2016 9:45pm

Short answer, Yes, it can.
But as stated a pack can still last years given voltage sag, but decreasing the sag for any particular battery would increase it's net life span.
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Re: Is Voltage sag damaging to batteries?

Post by Sunder » Nov 17 2016 9:57pm

dogman dan wrote:It would be better of course, to have less sag. I often put 30 ah of battery on my bike, just so I can enjoy nearly zero sag. Bike rides much better, perkier, doesn't slow much till I've ridden over 30 miles. lots nicer. But I am carrying 25 pounds of battery. This works for me because the bike is built specifically to carry a lot of battery.
Have you ever thought of using power-dense lithium titanate, to back up your primary battery?

LTO has a flat discharge curve, so you don't lose "perkiness" until the battery is almost flat. They can charge and discharge at over at over 150C, so they're ideal for buffering propping up the the voltage during high draw, then quickly absorbing energy from the main back during low draw.

I wanted to do this, but I could only find one seller of the 150C types, and he took over a week to reply to an email. By that time, I assumed he wasn't interested in small quantities and bought LiPo instead. (To stiffen up old but large LiFePo4).
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Re: Is Voltage sag damaging to batteries?

Post by izeman » Nov 18 2016 7:51am

Mündawg wrote:Short answer, Yes, it can.
no, i wouldn't say that's true per se. it's not the sag that's killing the battery. voltage sag is every battery's true nature. it's expected to sag. the question is: how much does it sag, and how low does it sag. as mentioned before: if it sags too much, than it's a clear sign that the internal resistance of the battery is too high. and high resistance means more heat.
if you sag makes the cell's voltage go below it's low voltage limit you will damage the battery as well.

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Re: Is Voltage sag damaging to batteries?

Post by Mündawg » Nov 18 2016 9:16pm

izeman wrote:
Mündawg wrote:Short answer, Yes, it can.
no, i wouldn't say that's true per se. it's not the sag that's killing the battery. voltage sag is every battery's true nature. it's expected to sag. the question is: how much does it sag, and how low does it sag. as mentioned before: if it sags too much, than it's a clear sign that the internal resistance of the battery is too high. and high resistance means more heat.
if you sag makes the cell's voltage go below it's low voltage limit you will damage the battery as well.
That's why it was the short answer :D
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