Circuit design for charging and discharging dual NiMH batteries

ena

Circuit design for charging and discharging dual NiMH batteries [Copy link]

 

What I am working on now is a digital display device, which uses a 12864 screen and an operating voltage between 4.5V and 5V. The circuit is very simple, controlled by a single-chip microcomputer and driven by a 595 extended data line. Now the customer requires the display to be displayed for more than 15 minutes after power failure (I blamed it for not considering it in the early stage), but I measured the current and it was about 25ma. Now I am going to use two 3.6V nickel-metal hydride batteries plus a DC-DC circuit. When charging, they are charged individually, and when discharging, they are connected in series. But how do I make the charging and discharging circuit? The module is powered by 5V

ena

Urgently needed!!!

chunyang

When batteries are used in series, they should be charged as a whole. If they are charged separately, the circuit will be much more complicated. NiMH batteries are charged with a constant current using a current source circuit, and then the voltage at the battery pack terminal is monitored using an ADC or comparator. When the voltage reaches the limit, trickle charging can be performed. It is best to use lithium batteries as backup power sources. There are many lithium battery charging management chips now, which shorten the development time and reduce the overall cost.

ena

chunyang posted on 2019-8-5 16:29 When batteries are used in series, they should be charged as a whole. If they are charged separately, the circuit will be much more complicated. NiMH batteries are charged with a constant current using a current source circuit, and then an ADC or comparator is used...

The battery is a button-type battery connected in series, 5 cells, 6V voltage. I found on the Internet that there are more charging ICs for lithium batteries and fewer for nickel-metal hydride batteries.

chunyang

ena posted on 2019-8-10 17:28 The battery is a button-type battery connected in series, 5 cells, the voltage is 6V. I found that there are more charging ICs for lithium batteries on the Internet, and fewer for nickel-metal hydride batteries

Charging NiMH batteries is much simpler, so no charging IC is required.

qwqwqw2088

There are many domestically produced lithium battery charging ICs

No need to use NiMH

qwqwqw2088

The voltage of lithium battery is 2.7-4.2V. It needs to be charged by the charging circuit to reduce the voltage from 5V to about 4.2V, just like charging a mobile phone.

When discharging, because the landlord needs the working voltage to be between 4.5V-5V, a boost circuit should be added to increase the battery voltage to 5V

ena

chunyang posted on 2019-8-10 17:50 Charging NiMH batteries is much simpler, so no charging IC is needed.

Well, I plan to charge it directly with 5V power in series with a resistor, and then use a boost chip. Isn't the charging current the initial voltage divided by the resistance?

ena

qwqwqw2088 posted on 2019-8-10 17:55 The voltage of lithium battery is 2.7--4.2V. It needs to be charged by the charging circuit to reduce the 5V to about 4.2V. Just like charging a mobile phone, when discharging, because of the building...

I read online that lithium batteries are not very stable, so I didn't dare to use them.

chunyang

ena posted on 2019-8-10 23:26 Well, if you plan to directly charge with 5V power in series with a resistor, and then use a boost chip, isn't the charging current the voltage at the beginning divided by the resistance?

The resistance value selection of the series resistance method is not based on the initial charging current, but only on the long-term steady-state current, that is, only the trickle charging current is considered to compensate for the self-discharge loss, otherwise it will seriously affect the battery life. However, whether this method is suitable for your application requires an examination of the battery discharge cycle. It is not suitable for applications that require frequent large current discharges, and is only suitable for applications with micro-discharge currents such as power-off data retention. Obviously, your application is not a micro-current discharge type, unless your application rarely uses a backup power supply, but judging from your description, it is obviously not. Therefore, honestly consider a regular charging solution. Trying to save trouble will only cause trouble later.

chunyang

ena posted on 2019-8-10 23:27 I read online that lithium batteries are not very stable, so I didn’t dare to use them

Why? Stability or instability is determined by design, except in some extreme application environments.

ena

chunyang posted on 2019-8-11 14:39 Why? Stability or instability is determined by design, unless it is in some extreme application environment.

The trickle charge time is a bit long. This is used on the gas pump. The function is a power-off display function. I used 3.6V/60mah before. The charging method is 5V in series with a 100-ohm resistor. As a result, leakage occurred after a long time. Another question is, if it is a 4.8V battery, the full-charge voltage is 5.6V. I charge it at a constant voltage of 4.8V. Is this okay? Will it cause the battery to not charge? Thank you!

chunyang

ena posted on 2019-8-11 21:45 The trickle charging time is a bit long. This is used on the gas pump. The function is a power failure display function. I used to use 3.6V/60mah, ...

If you only use resistors, you can only use trickle charging, otherwise the battery will die quickly. Constant voltage charging is better, but if the battery is deeply discharged, the initial charging current is likely to be too large. Moreover, if the constant voltage point is not selected properly, either the battery cannot be fully charged or the current is not appropriate, both of which affect the battery life or capacity. If you want long-term maintenance-free operation, you should honestly use the method of constant current to constant voltage and finally trickle charging. The process and circuit are ancient and mature, and there is nothing else to say.

伟林电源

ena posted on 2019-8-10 23:27 I read online that lithium batteries are not very stable, so I didn’t dare to use them

Just use polymer.

伟林电源

No matter what kind of battery you use, you should consider adding two more circuits.