About the balancing, the bq77910a manage protection and balancing for 10 cells. I thought I only need to provide 42V and 2A to manage the charge.
Yep, Sorry for this misunderstanding. In my mind I did a split for the charging and the protection circuit. I'm newbiesYou should have mentioned you were using the bq77910a in the first place that changes everything! You can't expect useful responses if you don't provide all the information! Your original post said absolutely nothing about the IC you were planning on using.
I'm building an open source electric bicycle with a ultra small package and small weight. I decided to start with the battery to control the size, the layout and the charge.To what extreme's you need to take charge/discharge considerations into effect is determined by the application itself which you have described nothing about
This can not occur, the protection IC would prevent it.ChrisP58 said:you could end up damaging a deeply discharged pack, instead of charging it.
You're not keeping abreast of his previous posts, the configuration you recommended is not required because the charge control IC he's selected already does all the work for balancing during both charge and discharge cycles and functions as both high/low voltage shut off, over current, short circuit, and even thermal overload protection. It doesn't do anything sophisticated for charging, however if it is programmed with conservative timeout reset values for the over voltage shut off, it will allow charging from a constant current source that's rated for a few volts over what is required. Once the voltage goes 'over' which would actually be set to the 80% charge voltage of 4.1 it would shut the pack off for a long delay, it would periodically restart and repeat the charge to 4.1 volts and shut off again, it would perpetually keep the pack at an 80-85% charge state which is ideal for Lithium Cell lifespan. You lose about 20% of the cells total capacity for discharge but this prolongs pack life and requires no advanced charging ciruitry.WTP Pepper said:He does need isolation if he chooses to use the configuration I posted.
20 x NCR18650A from panasonic. That are the best in term of Wh/kg.What cells are you using for the battery? That will be the defining point. You can not use cheap cells in a pack even with a ballancing cirucuit.
One thing to remember about boost converters is that they have no control when the output voltage is less than the input. And while your 36 volt pack should never be less than 30 volts, there may be fault conditions where it could be. If you don't provide some other means to block the current from the source supply, you could end up damaging a deeply discharged pack, instead of charging it.
The simplest is to use a buck converter that is higher than the 42 volt target, or use a converter topology that allows a controlled voltage both above and below the input voltage. A SEPIC converter would work, or a single inductor buck-boost converter. See TIs LM5118 or Linears LTC3789 for examples of the latter.
I have no religion for the implementation except I want to use common power source of laptops. They provide good AC conversion and current limitation (24V6A). I guess that some of your advises don't concern my needs. First I don't have other function powered by my supply on the charge, and the current limitation is already done by the laptop power supply. Am I wrong? What is the *correct* implementation? Is the suggestion from Sceadwian a good solution?As a result, the open-frame 24V supply that I was using to run the charger would go into hick-up mode when a deeply discharged pack was connected. Since there were other functions powered by that supply, they all shut down too. Eventually, the high current pulses between start up and current limit did charge the battery up to the point where the boost converter could control it, but it was bothersome to the end user as the lights on the panel kept going on and off as the uC kept going through it's start up cycle.
In your case, the protection circuit in the battery will prevent catastrophic failure due to over current charging, but that doesn't mean 'soft' damage or degradation won't happen if it is exposed to short. repetitive current pulses of uncontrolled magnitude.
Can I power all these chargers in parallel of the protection circuit? And how do I isolate the output of the dcdc convertor?Sceadwian said:Build a simple single cell charger using one of many Li charger ICs (MAX1811 comes to mind but there are others), but power it from a DC-DC convertor (5v output) that has an isolated output. These standalone small modules are readily available.
Power all of these chargers in parallel, but series wire the cell output.
Use the LED output from the individual charger ICs to monitor the whole pack and spot early failures of individual cells.
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