It gets complicated if you don't always stay in the same country. What if you topped up £10, used £7.29 of it, then went to the US and topped up $10? Should you

It gets complicated if you don't always stay in the same country. What if you topped up £10, used £7.29 of it, then went to the US and topped up $10? Should your balance be £2.71 + $10? Which currency should be used when you buy things? How should conversion be done? I understand the motivation of wanting a separate currency.