when we are driving, we need know the level charge in the batteries - the industry is providing a device for put in the panel ? better yet, put in the level gas meter place, joint the velocimeter
This is notoriously tricky with batteries. The voltage - being more or less constant during the discharge cycle, except that it varies with temperature and current drawn, as well as with age - is not a good indication. You can measure the energy in and the energy out and this gives you some idea, but as batteries age this becomes unreliable too, and again temperature comes into it. With smart software and temperature sensors etc. a microprocessor can improve the accuracy quite a bit but it remains a rather rough and ready indictor. Capacitors are much easier. The energy remaining (the 'gas' in the tank) is directly proportional to the voltage squared. So you just measure the voltage and square it if you want a nice linear indication. This is easy to do with a microprocesor, or with an analogue 2 (or 4) quadrant multiplier chip and you can have the indication in Joules, Watt-Hours - whatever you want.
You just need to count amp hours. A lot of people use a JLD404 for this. Here's one on the EV West website.
This is measuring how much you take out I think, rather than telling you what's left. This may be good enough in practice. But batteries are designed to give you a constant voltage for as long as possible and can, therefore, give you little warning that they are about to run out. I would imagine that the best you could do would be to measure how much energy goes into your battery, knock off 20% or whatever you expect to lose at the charging rate you are using, and then by monitoring what comes out of the battery you have an indication of what you have left. This is easy to do with a microprocessor, which could probably incorporate an adaptive algorithm that learns what the actual losses are as the battery ages to improve its accuracy. Its a bit surprising that this or something like it isn't built into cars anyway. I know in ICEs a microprocessor is used to average out the changes in level of the fuel in your tank and to compensate for the fact that the tank can be an odd shape so that the level measurement is not linearly related to the quantity of fuel anyway. It seems to me that this is a task of comparable complexity.
