EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: V_King on September 08, 2017, 06:50:15 am
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I've been playing with deep cycle lead acid batteries recently to power my UPS units, working as inverters.
What I found was a very old school approach to the battery bank maintenance. I had three deep cycle used batteries from my boat and needed to add another two, but because of the age of the older ones, I was advised to replace the whole battery bank to make sure batteries are in same condition. I found that to be very wasteful, especially when buying good quality deep cycle batteries.
And it got me thinking. All the advanced battery technologies have proper cell controllers, which control charging, discharging and general health of the battery. I tried googling for such controllers for lead acid batteries, but all I got was charging controllers in all kinds of shapes and sizes. I wonder if it would be hard to build a lead acid battery controller, which would do the following:
1. provide charging of the battery to the lead acid battery requirements
2. provide battery maintenance (eg keeping it topped up as long as the power to the charger is applied)
3. provide a constant 12V DC output no matter what voltage the battery actually is. I think this bit is very important, as the deep cycle battery tech data sheets I've been reading showed that the 50% discharge was always above 12V without load.
4. turn off the battery if it reaches the set depth of discharge rate.
Then, each battery would have a controller as per above, mounted on each battery on top probably, and then it makes the whole battery bank maintenance so much easier. And it would not matter what brand, state or quality the battery is behind the controller, for the overall system.
I have experience with digital electronics, but analogue power electronics are not my strong point. I am sure I could find of the shelf IC for the battery controller and maintenance side, but for the high current 12VDC side I have very limited idea of what to do. High current mosfets/IGBTs with high frequency switching and some ADC to monitor the voltages? If somebody could point me to the right direction for reading/tutorials (practical side, not dry theory) I would much appreciate.
I recently pulled apart my inverter welder, which could do 160A at 100 duty cycle and was amazed of the compactness of the whole thing, so I think for my proposed controller the affordable technology is here.
Any other thoughts?
Thanks,
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You could do all of the above and I have considered it in the past however ...
1. provide charging of the battery to the lead acid battery requirements
2. provide battery maintenance (eg keeping it topped up as long as the power to the charger is applied)
Then, each battery would have a controller as per above, mounted on each battery on top probably, and then it makes the whole battery bank maintenance so much easier. And it would not matter what brand, state or quality the battery is behind the controller, for the overall system.
6 and 12 volt batteries do not provide access to individual cells so balancing and monitoring can only be implemented for groups of cells. This does not apply to 2 volt cells however they are only found in much larger installations.
PbSO4 cells, especially flooded cells, may be balanced by a controlled overcharge which is also desirable anyway to prevent electrolyte stratification in stationary applications.
3. provide a constant 12V DC output no matter what voltage the battery actually is. I think this bit is very important, as the deep cycle battery tech data sheets I've been reading showed that the 50% discharge was always above 12V without load.
4. turn off the battery if it reaches the set depth of discharge rate.
Both of these things could be done. The second is more important to prevent damaging the cells.
A buck-boost switching regulator (1) could provide a constant output voltage however loads intended to run off of 12 volt systems are already designed to handle the voltage variation do to charge and discharge so in most cases, it would just be extra complexity yielding lower efficiency and lower reliability. Converting 12 volts to 12 volts would also be expensive at high currents. (2) A better option would be to convert the output from a 48 volt battery bank to 12 volts.
In contrast, depth of discharge control is almost trivial to implement even at high currents.
(1) I am not referring to the literal buck-boost topology but to the general class of switching regulators which can provide a constant output voltage with an input voltage which varies above and below the output voltage of which there are several.
(2) This is an almost intractable problem at 12 volts because at the high currents PbSO4 batteries support, parasitic inductance becomes a major problem. Some type of current sharing "power module" would be the way to go where multiple units may be placed in parallel rather than trying to do it with one converter. This would also allow scaling from low to high power while retaining an economy of scale.