Ventura County, in California, doesn't sound really cold. I see about 30% drop in capacity if the battery is close to freezing.
In perspective, other bigger name bike sellers charge $600-800 for batteries. While Ecotric does use a third party generic battery, you can buy them from someone else and save maybe 100 bucks. You do want to avoid the real cheap batteries. That 48V 20AH $100 wunder battery, take my word for it, it's junk, 6AH at best.
Does your Hammer still uses this pack, it's called a Silverfish.
Description Condition: Brand new Package includes: Battery+keys Item size: 430*110*75mm Discharging position:Bottom-2 ports output Energy:624Wh Voltage:48V Case Material: Aluminium alloy+plastic Battery Type: Lithium lon Model: Seat tube battery Weight: 4.7kg Working temperature for charge...
www.ecotric.com
Anyway, there are lots of 48V silverfish packs. The two variables that matter are first, the bottom connector has to match. Some makers flip the polarity on the pins. Disaster if it happens. The second is the battery height, because of the way the battery locks onto the frame bracket, but the packs are often sold with the right bracket. Here's a listing just for reference. I have no idea of quality.
Find many great new & used options and get the best deals for SilverFish 48V 13Ah 750W Electric Bicycle lithium Battery Pack Charger E-bike at the best online prices at eBay! Free shipping for many products!
www.ebay.com
If it were me, I'd contact Unit PAck Power, another ebay seller and ask them to quote on a 48V15-17AH Silverfish using Samsung GA cells. I've had them sell me two generic packs with the Samsung upgrade. Costs more. Makes quite a difference.
www.ebay.com
FYI. I own the 20" 36V Ecotric fat tire folder. Inexpensive bike, but pretty good for the $700 it cost me.