'94 C280 odometer not storing accumulated mileage

[atikovi]

I've just put this car on the road after it's been sitting a few years. I write down the odometer reading when getting gas and put in $10 last week. I know I drove it at least 40 miles this weekend but the odometer reads less then then when I filled up. When driving the mileage does register and goes up. I do disconnect the battery after driving out of habit because it may be weeks before I drive it again but that shouldn't affect the odometer memory. Does the instrument cluster have some internal battery that needs replacing?