>> What would be useful to me, as a low mileage driver, is a gauge to
>> show when my battery needs a charge.
That used to be a "VOLTS" meter. Imperfect but a typical sealed lead acid battery reaches 12.8-12.9 when fully charged, then it will be in need of building up again if it drops below ~12.3. I just about remember using one mainly to check that there was some charging going on, rather than to guess at the state of the battery.
Search Amazon for 'volt meter 12v', there is a selection of stuff you can panel mount/stick on and wire in or plug into the accessory socket. Read it when it has no appreciable load and isn't charging.
If you search 'battery state of charge meter 12v' there are similar looking ones with a bar graph of the charge level. These are basically voltmeters but the slightly smarter ones can more or less calibrate themselves to a full charge.
Beyond that are battery monitors that measure Ah drawn to give an estimate of remaining charge. These also reset when fully charged and making an allowance for charging efficiency. More trouble than they are worth IME.
You have to be prepared for things costing a few pounds be to flaky so personally I wouldn't be cutting any holes in the dash for any of these.
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