I want to leave the folding ebike at the seasonal house and take the battery and charger with me south. Won't leave it in the deep freeze barn.
So I will have the battery, and charger, and will store battery at room temp in FL until next July. I've read that it is best to keep it between 50 and 75 percent charged, and definitely do not leave it on the charger.
So I guess I will need to juice it a little every couple months.
Can a multimeter tell me what is going on with it?
Very wise to take the battery with you. One option is to purchase a charger that you can set to bring your battery up to the voltage of your choice, such as the Cycle Satiator. That's very effective and convenient, but expensive! Using a multi-meter is probably the better choice for most bikers. This chart should help.
We all recognize that the battery is a very important e-bike component, so it is definitely worth the extra effort to take good care of it. Get the most value for your dollar by extending the lifespan of your battery. For your winter storage, bring your battery up to about 75%, and then it will be fine for quite a long time, Bringing it back to 75% once every 2 months is normally adequate.
When you have returned and are using your e-bike again, every time you recharge your battery, try to stop the charging process at no more than 90-95%. The exception is if you have a new unused battery. For the first 4 or 5 charges on a new battery, go all the way to 100%. After that, routinely charge it to 90-95%. This prevents the electrical process of charging from doing any damage to your battery. It is a precaution that will add many months of battery life. However, develop a once-a-month habit to do just one recharge to 100%. To make it easy to remember, try doing the 100% charge for your first ride of each month. I own 3 Magicycle Cruisers, and ride almost every day. It doesn't take long to become familiar with your battery and get in the habit of limiting the charge.
As much as possible, avoid discharging your battery to less than a 20% charge. Over-discharging stimulates oxidation inside your battery. In most cases, this is not necessary, and it will shorten the life of your battery. Also, regularly over-discharging will gradually reduce the capacity of your battery, forcing you into shorter rides.
All batteries develop heat during use. They also develop heat during the charging process. It is important to never begin charging immediately after returning from a ride. Your battery already contains heat from the ride, and it needs to cool for at least 30 minutes before you begin charging.
Finally, i is never safe to leave your battery unattended during the charging process. You may have seen or read horror stories about batteries overheating during charging. The result can tragically be a serious fire, explosion, or both. Check on your battery intermittently while it is charging. If you simply cannot monitor the charging, then consider doing your charging outside and nowhere near anything that could be harmed in an explosion or fire.
Hope that helps, and have a great winter!