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I have a 120 V / 60 Hz 2000 watt inverter which can draw up to 200 amps from a 12 V DC battery source. An AC oil heater drawing 6 amps max 15 amps is running from the inverter constantly. I want to automatically change batteries without interrupting the current to the inverter.

The scenario is: several batteries are supplying the inverter. When a battery becomes depleted it is replaced with a charged battery. The invertor cannot go off line so a battery must be on line at all times. The depleted battery is switched to a charging circuit until it is fully charged then it is available to be switched to the inverter load. The battery being charged must be isolated from the inverter during charging.

Having many batteries is not an issue. Having many battery chargers is not an issue.

Currently this is a manual process and I'd like it to be automatic. I'm looking for: the best way to detect the depleted battery, swap the battery from the inveter to the charge circuit, detect the fully charged state then return the battery to the load circuit.

This application is an off grid. Any advice will be helpful.

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    \$\begingroup\$ Two reasons. 1) the inverter is under a constant load and will over run the charger and ultimately deplete the battery. 2) the charger is a pulse tpye and needs the batteries to be isolated and not under a load \$\endgroup\$ Jan 7 '16 at 12:54
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    \$\begingroup\$ Thanks, @Dan. These details need to be in your question then. So does info on charging / running duty cycles and times. Improve the question and the answers will improve too. Link to the charger and inverter data sheet. \$\endgroup\$
    – Transistor
    Jan 7 '16 at 12:59
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    \$\begingroup\$ If the inverter can "overrun" the charger, meaning that the inverter takes more power than the charger, then you have a fundamental problem anyway. It doesn't make any difference whether the batteries are charge on line or off line. You're still going to run out of charged batteries. \$\endgroup\$ Jan 7 '16 at 14:03
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    \$\begingroup\$ @Whisk: Either way, this is not a reason the charger can't be connected to the batteries while they are connected to the inverter. The charger can either keep up with the inverter or it can't. Charging the batteries disconnected won't change that. \$\endgroup\$ Jan 7 '16 at 14:11
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    \$\begingroup\$ @DanErickson, nobody has jumped in with an answer. Take this as a sign that we're all uncomfortable with your approach to the problem. If you answer our comments (put the details in your question rather than in the comments) and can convince us that your approach is the best under the circumstances then you will get better answers. \$\endgroup\$
    – Transistor
    Jan 7 '16 at 14:22
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There might be better ways to do this but we need more details on the topic.

However one possible way around this is Voltage sensor + high current Mosfet switch.

Feel free to use a micro-controller of your choice. Make sure it has adc capabilities(almost all have). You job will be simple if it has as many adc pins as the number of batteries you are planning to switch between. If that's not possible, you can always use multiplexers to do the job.

Voltage sensing: Make a voltage divider to bring the charged battery voltage within range that can be measured by your micro-controller (5V, 3.3V or whatever your adc pin allows). Make sure your adc pin never sees a value which is out of range. For ex - If battery has a max voltage of 15V and my adc can accept max voltage of 5V, I can use this circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

This will allow you to sense the battery voltages. You will know which batteries are ready for getting fed into the inverter.

For switching, you need a high current mosfet - one for each battery. Parametric search on digikey will give you some results. If your max current can be 200A, better to select a value higher than that - probably 2-3 times.

Another solution (cheaper) is electromechanical relay but at that current, arcing might kill it very often.

If you are using mosfets, you will have heating issues so you will definitely need to attach massive heatsinks to your mosfets. Having a common heatsink for your mosfets might help because only one mosfet will remain ON at any particular time. Due to the big size, cooling will be more effective as compared to a small detached one. Assume a power dissipation of 150-200 watts in your mosfet while carrying 200A. If that's a concern, you probably shouldn't go this way.

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  • \$\begingroup\$ Thank you Whiskeyjack, this was one of the approaches I had in mind. \$\endgroup\$ Jan 7 '16 at 15:03
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As others have already noted in comments to your question, you have a fundamental problem in your setup in that you are using more power than you can supply.
Even assuming you start off with a bank of fully charged batteries all ready to be swapped in, no matter how many batteries you have you are only delaying the inevitable.
If you can't charge a battery from empty to full in the same or less time it takes to drain it from full to empty then you will only end up with more & more batteries waiting in the queue to be charged - eventually leading to a bank of empty batteries with one being charged and none available to be used.

No, this doesn't answer the actual generic question, but addresses the issue which really makes it unanswerable in your specific application.

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There are so many problems here. First, it is always better to have a single (or maybe two) large battery bank than many small ones. Because then you will be discharging the individual cells more slowly, which means the discharge time will be longer, and the number of discharge cycles over cell lifetime will be greater. This is probably the main reason people do not switch individual cells in and out in an engineered system.

As others have pointed out, if the charger cannot supply enough power to run the load, then eventually you are going to run out of batteries anyway. But to delay that eventuality as long as possible, you want to use a large battery bank, not a bunch of small ones.

I imagine that you should be able to switch batteries using MOSFET's or IGBT's. The max current rating should be higher than 200A and probably the highest you can find available, but I don't normally design power electronics. You will want to connect the new battery prior to disconnecting the old one, but the interval where both batteries are connected to the load should be kept as short as practical, because the "good" battery will be running the load AND charging the bad battery with a very high current. Each battery should have a fuse in case something goes wrong with the switching scheme.

You can use a simple voltage threshold to detect depleted batteries. Deciding on the best endpoint would require quite a bit of knowledge and work. If you insist on your ill-advised plan, then you will be discharging a single battery at over 200 Amps, so you can probably consider the battery depleted at 10V. If you create a large battery bank that can supply 200A for many hours, you could consider using a higher voltage (maybe 10.5?) as your depletion threshold.

Basically, the faster you discharge the battery (as a proportion of its capacity), the lower the voltage will be when the battery is at 80% (or whatever) discharge. So the heavier the load, the lower the threshold.

Most 12V batteries are not designed to supply 200 Amps continuously. I don't know what kind of batteries you are planning to use, but I am skeptical that they will last for very many charge/discharge cycles with this type of use.

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