I sold my 2002 GEM this year after owning it for 19 years. I am now the proud owner of a 2025 GEM e4 with a factory 8kW lithium battery, and I love it! I am a retired NASA mechanical design engineer, but have limited experience with Li-ion batteries. My current thinking is to treat the GEM car batteries like I treat my other Li-ion devices, such as cell phones, leaf blowers, and lawn mowers. I generally try to charge them only to 80%, and discharge them no further than 30%, in order to get maximum service life out of the batteries between replacement.
The GEM ownerâs manual only speaks in generalities regarding best practices for batteries, while not specifying whether their advice refers to Lead Acid, AGM, or Lithium-ion. They do not address lithium batteries specifically, so I am reluctant to trust their advice.
Anyway, the ownerâs manual says to always charge the battery to 100% after every use. They advise to avoid âshort charging,â or disconnecting the charger prior to full charge. They say this will drastically reduce battery life. This advice is completely contrary to my understanding of how to maximize service life of lithium batteries.
My question for the forum experts on Lithium batteries, is how should I charge/discharge my GEM 8kW Li-ion battery to maximize service life in years?
Li batteries like full charge to use them and 50% charge to store them. They can take a deep cycle discharge without bad things happening, unlike lead acid types.
It depends on the chemistry of the Lithium battery but for non-LiFePo4/LFP batteries 30%-80% is better than going to 100% unless you use the GEM when it hits 100%. The research Iâve seen shows that itâs optimal to do more 30-40% charges to 80% than fewer higher capacity change charges. Something to do with molecular fracturing from heating and expansion.
LiFePo4/LFP battery chemistry is different and you can charge to 100%(if the BMS doesnât really take the cells to 3.65V max) and deeply discharge to as low as 10%. And it doesnât matter if you charge 50%, 80% or even 90% in one charge session.
Iâve got 2 GEMs and 2 full size EVs with âstandard"â lithium chemistry and a 50kWh whole-house battery made of 3 DIY LFP battery packs.
Thank you all for the detailed answers. I do have a follow-up question:
I store the GEM for 5 months in the winter, unattended. I have several options:
The GEM factory recommends leaving it plugged in for storage periods up to six months. The on-board charger will bump up the batteries each time the charge drops below 97%. I donât like this option, as it is a blind instruction and does not differentiate between Lead Acid, AGM, or Li-ion batteries. Long-term storage of a fully charged Li-ion battery puts undue stress on the battery, and can permanently damage it.
Leave battery at 50% state of charge and disconnect battery by setting the storage switch under the charge door to the OFF position. Recharge it 5 months later. Garage stays around 40 degrees F. I donât think the battery will discharge further when turned OFF, but I donât know if the GEM designers left in any parasitic draws. The storage switch appears to control a solenoid that disconnects the battery.
Do option 2, but have a neighbor check on the battery after a couple of months, and bump it up to 50% if necessary.
i vote for door #3 because it would be an expensive lesson to find out something caused the battery to go too low. Atleast the first year.
Option 1 is a terrible idea and if that is what they say in the manual for a product which does not sell with lead acid batteries, they should be told immediately to pull it down because it could be dangerous if anything failed in the electronics or battery chemistry since internal shorts grow at those potentials.
Does your '25 have a little pony/start battery? Donât forget about that little thing. Once it goes dead your car wonât start.
My first reflex for long term storage would be to unhook a leg off each battery if the car was not going to be used for a while (to eliminate any possible parasitic drain) but Iâm not sure if that would cause any issue with memory systems on the newer cars. It shouldnât, but since I donât have one I donât have any way to test it.
Iâm not really sure whether it has a pony/start battery. I do have a maintenance manual, so Iâll have to check it out.
We are leaving for a couple weeks, so I am going to turn off the GEM battery at 60% state of charge, just to monitor whether there is any parasitic amp draw over two weeks time when the battery is switched off at the battery door.
Iâm not really sure whether it has a pony/start battery.
Figure out how to pull up dash panel and have a look.
Rather than trusting the dash display, you should take a direct read off the battery. Write down that number before and after.
Now that I think about it⌠Is this battery Lifepo4? A V reading may not even be the best way to calculate battery SOC.
When the car is fully shut down, the onboard SOC may not see any stray current and properly derate the bar graph properly.
2025 doesnât have the pony battery. Itâs a whole different animal. WAVE tried to undo everything that Polaris did. Different controller, different Charger, different DC DC converter, different layout of components.
The DC DC converter is always on which eliminates the need for the pony battery but puts on a big parasitic draw. Thatâs why they added the cutout switch. Canât store it for 2 weeks without killing batteries unless you throw the cutout switch.
Thank you for the additional info and explanation. You guys are a wealth of information! I did verify that the battery chemistry is Li-ion, and there is no pony battery beneath the dash.
I do try to wait 3 or 4 hours after driving the car before putting it on charge. My understanding is that it is less stressful on the batteries to let them cool off before recharging them.
I have only had one issue with the car since I bought it in June. The transaxle had an oil leak show up after a couple of months. It turned out to be overflow out of the high point atmospheric vent pipe stub on the transaxle. I assume due to warm oil expanding while driving the car. My dealer rigged up a rubber tube vent hose to a higher elevation Problem solved. The manufacturer Waev has not designed a retrofit fix for the known issue yet, but it will be a similar pigtail vent tube.
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