tackling a logistical task.

[sergbot]

So I've got a 12 volt car battery.
When the car battery is fully charged, it will fall down to 12.6 volts.
When the car battery is charging, it will be charged by 14 volts

My goal is to make the battery be only half charged, so i may charge it with something else.

I was actually thinking about using lithium ion cells because they will have a high nominal current, preventing the 14 volts from doing that much charging, and then the other power supply can have higher voltage.

or i was thinking about getting seven counts of two volt lead batteries, this will raise the battery voltage and the battery won't get charged by the now too low 14 volts.


Another problem is, what happens if I hook up two power sources in parallel. One has low voltage but high amperage. Other has 2 more volts but not as many amps. I'm don't know if the high voltage source will make the overall voltage higher when everything is connected, and then the amps from the first power source, whom aren't supposed to be used, will fly into the battery.

So should I put diodes and stuff to direct the flow of current to stop the possible high system voltage, or should i just put in relays that will control when is what connected. Also how am I going to get the main battery voltage if its under voltage and current and is at 14 volts?

[MorganS]

What? Why?

I think I understand the second part of the question so I will answer this.

what happens if I hook up two power sources in parallel. One has low voltage but high amperage. Other has 2 more volts but not as many amps.

This depends on the behavior of the higher supply. Usually a charger will provide a limited current. That means the voltage is variable. So the higher voltage charger will be dragged down to the battery voltage. It will charge as hard as it can (max current) until the voltage comes up. You can supplement that charger with a second one with a lower voltage. That will deliver the current required to bring the battery up to its desired voltage. That current will taper off to zero as the battery charges. It should hopefully remain zero as the slower charger exceeds the fast charger voltage.

You may notice that I switched from "supply" to "charger" above. What we normally consider to be a "supply" is fixed voltage, not fixed current. In that case the higher voltage one might be destroyed by the overload condition.

The other problem you might find is if you try to connect a 13.8V smart charger along with a 14.1V smart charger is the two smarts will fight each other.

What are you really trying to do?

[sergbot]

Hi. I have one charger and one generator. The generator is 14v, the charger will have a boost converter so doesn't matter what v, we can just set it. I want the generator to charge the battery only if its under 50% capacity. Then I want the charger to finish the other 51% to 100%.

Does an electric motor generating electricity also have the same characteristics as you mentioned power supplies have?

Also I saw you talking about the lower voltage charger reaching the battery voltage. Will this happen even though the lower "charger" has max voltage of 14v, the lower charger can still hit higher chargers voltage?

sorry for making it so confusing.

[MorganS]

Ok. That sounds better already.

You have a generator that you want to use to charge up to half way. This is like an emergency when the battery gets really low. But you don't want the generator to run all the time.

So what is the other charger? Wind? Solar?

[sergbot]

yup. solar charge controller.
the issue is that both the solar charge controller, the generator, and the battery are always connected in parallel, and the generator always spins.

[MorganS]

So shut down the generator when the battery voltage exceeds 13.6 (or whatever) and only switch it back on when the voltage falls below 12.1 (or some other value you choose through experience.)

[sergbot]

Can't really "shut" it down. Can disconnect it with a relay though. But how do I know when to disconnect the generator? it causes the battery voltage to always be 14v. I can't tell the battery voltage when the generator is connected.

Also lead batteries like to drop in voltage slowly after disconnecting. It will take 1-2 minutes for the lead battery to finally reveal the voltage it's really at. it will be dropping from 13.8 volts for 2 minutes.

[MorganS]

So the generator is going to run all the time? Why do you need the solar then?

If the battery is actually flat (or below half) then you won't see 14.0V on the battery terminals. It will be lower.

[jremington]

To estimate the charge level of a lead acid battery, the usual way is to let it sit for a few minutes, disconnected, then measure the voltage.

There are other, far more complicated and often less reliable possibilities.

Excellent info on lead acid batteries here:
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

[MorganS]

Also lead batteries like to drop in voltage slowly after disconnecting. It will take 1-2 minutes for the lead battery to finally reveal the voltage it's really at. it will be dropping from 13.8 volts for 2 minutes.

That is a good thing. You don't want the generator to go on and off 5 times per second. A 2-minute rest sounds like a good minimum.

[Paul__B]

So you need to explain - why if the generator is always running do you want to disconnect it from the battery?

Petrol (and similar) generators are extremely inefficient; It will consume fuel while ever it is running, it may not even make any measurable difference in the fuel consumption once the battery is mostly charged.

[sergbot]

So the generator is going to run all the time? Why do you need the solar then?

If the battery is actually flat (or below half) then you won't see 14.0V on the battery terminals. It will be lower.

the generator has a boost converter that will always bring up the system voltage to 14v regardless of the state of charge of the battery.

To estimate the charge level of a lead acid battery, the usual way is to let it sit for a few minutes, disconnected, then measure the voltage.

There are other, far more complicated and often less reliable possibilities.

Excellent info on lead acid batteries here:
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

Well :/ that's kinda wasteful, because that three minutes it could have been being discharged or being charged... Hmm what if we used lithium ion instead of lead? do we still have the same obstacle?

So you need to explain - why if the generator is always running do you want to disconnect it from the battery?

Petrol (and similar) generators are extremely inefficient; It will consume fuel while ever it is running, it may not even make any measurable difference in the fuel consumption once the battery is mostly charged.

So you need to explain - why if the generator is always running do you want to disconnect it from the battery?

Petrol (and similar) generators are extremely inefficient; It will consume fuel while ever it is running, it may not even make any measurable difference in the fuel consumption once the battery is mostly charged.

Well because it's a car and the generator is always running. lmao. Well i don't think that's entirely correct. When the generator is underload it will create massive resistance on the drive belt. When the generator is not connected, it will just spin freely. it will still cause friction because it exists, but it will be 50 times less friction compared to if it was under maximum load. I'm just guesstimating, don't quote me on that.

[MorganS]

You have a car? That is a huge upgrade since you started with only a car battery a few days ago.

Goodbye.

[123Splat]

you should measure current in the battery side. when the current is high, battery capacity (charge level) low. when the current is low, or almost nill, the battery capacity is high (charged).  approximate the midpoint and switch chargers at that point.  if you now have a car, take the voltage regulator circuit out of it and use that.  If you are thinking of replacing the voltage regulator in your car with arduino based circuit, don't. the system in the car will do a much better job and last longer (as it was designed for automotive use), and you won't end up with a dead battery when you need to start the car.

[wvmarle]

So we started with a battery and a generator.
Then a solar cell was added.
Now a car.
It really makes me wonder what more is missing from the whole picture.
You know, it works a lot better if you give the whole picture from the start, because now there will always be the question "what other important info are you keeping from us?"