What causes excessive voltage sag?


Well-Known Member
Norfolk, VA
Both of my Lectric ebikes get the worst voltage sag. I'll have a full charge, pop it in max PAS to cross a street and next thing I know I'm down to 4 bars remaining! Then when I stop at my destination I come back out and I'll be back to full charge. I thought it was because of the colder weather but it was a lovely 60's outside for the last few days and the sag was still prevalent. My XP suffers a bit more than my Xpedition dual battery but both 'Suffer from the Sag'. Is this common across all ebikes or are better quality batteries less prone to this phenomenon?

I don't recall my RadRunner suffering voltage sag like this, I recall only loosing a 2 bars max. Note that I had the basic display, with less battery bars, so maybe it was the same amount of sag? So, is this common across all ebikes or just cheaper batteries? And if it affects certain batteries over others how/what do I look for when shopping for my next ebike to avoid this issue?
Does your bike have a voltage readout? The bars aren't the most reliable gauge. Also, you could have a bad cell group. Severe sag would occur when a cell group goes bad, or (as stated above) the internal resistance gets higher the more the battery ages. What is your charging routine like? Leaving a battery plugged in for hours after it has been fully charged will weaken the cells.
I bought two $300 batteries that sagged badly on any hill, one from Amazon to 7 volts, the other from ebay to 11 volts. I got the money back on the Amazon one, it would stay at the low voltage until I disconnected it. The ebay one would snap back to 52 v as soon as I disconnected the load. Did not get my money back on that one. I had to devise a load test to prove whether the battery or the controller+motor was bad. 10 ohm 800 w resistor would sag the battery below 40 v every time. When I cut the top open and examined the bms, only one stack of 14 was actually connected. Not enough to even support 100 watts. Worked great as long as I only needed 50 watts. Probably would not have generated 800 watthhours if I had been able to ride that far on the flat (below 50 watts).
My $630 luna battery is still great at 4.5 years age and ~9000 miles. They must have actually welded the cells together.
So basically cheaper ebikes use cheaper batteries. That makes total sense. I'll pay attention to the voltage next time and see how bad/fast it drops. It makes my commute kinda annoying. For my next ebike, for commuting, I want to know what to look for in a battery before buying. I don't want another more premium ebike to sag like my Lectrics. I still love my Lectric ebikes but the battery sag is annoying.

Since I don't wanna give up my "T" word I was going to just get a ugly Globe Haul LT and replace all the ebikes. One and done. But did Specialized chump out on a quality battery? Reviews don't even mention stuff like battery sag. Or I could keep the Xpedition (it never goes far with the wife on the back) and get me something nice just for commuting. The Zen Photon is very tempting and I don't think they'll pull a Dost on us. Bigger wheels, mid-drive and the "T" word. I vowed to suffer 1500 miles on the sagging machine before thinking of a replacement.
The original Lectric XP's came with a 48V10.4Ah battery. Small, but it's a small bike as far as the motor is concerned, What you see is typical. The LCD bar gauge actually minimize the sag because it averages out the voltages, and has slower response. Do you have a voltage gauge on the XP?

It does get worse as the battery gets older. Where it gets you is when the sag is so large that it shuts the bike off when you're riding around below 50% charge, about two bars, and try to use throttle, If that's not happening yet, the battery is still useable. Keep riding.
Internal resistance of the batteries, the higher it is with worn out or cheap quality, the more the power is converted into heat, the hotter it gets the more resistance there is, vicious circle and all.
Internal resistance of batteries isn't really resistance. For example, a car battery has more internal resistance at 0 F than at 80 F, and that's because the cold electrolyte slows the ions swimming between plates. When lithium-ion batteries sag, I assume it's because plates are getting depleted of the potent chemicals, and they need time for those chemicals to make their way to the surface. When I got voltage sag on my Radrunner, it was apparently because some cells were much less charged than others. Balancing fixed it.