How much of time to full charge a 9V 800mAh battery

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
111
Dear Sir/Madam,

I have a 9V 800mAh rechargeable battery, this battery can constantly provide 9.18V all the time. It is a USB rechargeable battery. If I plug it to a USB mobile charger adapter, I measure the adapter providing 5.04-5.07V and 450mA to charge the battery, may I know how much of time the battery be fully charged? How to calculate the estimated time?

Best regards,

Kelvin.
 

ericgibbs

Joined Jan 29, 2010
11,890
hi K,
Making some assumptions, my best estimate would be:
The charger has a 1:2 step up from 5V to 10V, using a boost SMPS , say 100% efficient.
The measured current is 450mA at 5V, so thats roughly 225mA at 10V [ the assumed charging voltage]
Again assuming 100% electrochemical conversion efficiency of 100%, thats 800mA/225mA = 3.5Hours charge time for a fully depleted battery in good condition.
The actual SMPS Eff% could be 85% and the electrochemical conversion could be say 80%, giving a recharge time of just over 5 hours.

E
 

dl324

Joined Mar 30, 2015
12,442
I have a 9V 800mAh rechargeable battery, this battery can constantly provide 9.18V all the time.
"9V" rechargeable batteries will usually have a nominal voltage of 7.2V or 8.4V. When a battery with 7 cells is fully charged, it's voltage will be around 10.5V. When the battery discharges its voltage will drop.
 

MrSoftware

Joined Oct 29, 2013
1,970
There are several variables regarding how long it will take to charge, such as whether it can negotiate via USB for higher current (starting with USB 3 I think), the efficiency of the charger, the battery chemistry, does the charger take a long time to balance the cells (in the case of a multi cell lithium battery), etc.. Is it a commercial product and can you post the brand and model number?
 

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
111
I’d like to have one of those. But if that is true, why do you need to recharge it?

Bob
Dear Bob,

I use the rechargeable battery because the one-time battery is expensive.

Attached is the cell I bought, there are two kinds, this one always keeps at 9V even it almost discharged. I use this to my Arduino board. I tried some one-time battery purchased from the market when it drops down to around 6V then it almost cannot power up the board but this one has no such problem.


20190106_194358.jpg
 

Thread Starter

Kelvin Lee

Joined Oct 22, 2018
111
hi K,
Making some assumptions, my best estimate would be:
The charger has a 1:2 step up from 5V to 10V, using a boost SMPS , say 100% efficient.
The measured current is 450mA at 5V, so thats roughly 225mA at 10V [ the assumed charging voltage]
Again assuming 100% electrochemical conversion efficiency of 100%, thats 800mA/225mA = 3.5Hours charge time for a fully depleted battery in good condition.
The actual SMPS Eff% could be 85% and the electrochemical conversion could be say 80%, giving a recharge time of just over 5 hours.

E
Many thanks!
 

dl324

Joined Mar 30, 2015
12,442
There's a good analysis of this battery on youtube. As I suspected before even viewing it, the 800mAh rating is way optimistic.
I agree. It would be difficult to get 800mAh in that form factor. The 7.2V batteries in that form factor used something similar to small AAA batteries. To get 7 cells, they'd have to use something smaller.
 

dl324

Joined Mar 30, 2015
12,442
Attached is the cell I bought, there are two kinds, this one always keeps at 9V even it almost discharged.
No rechargeable battery is going to have a discharge profile like that. When fully charged, the cells will be above their nominal voltage. When they discharge, there's a period when the voltage is relatively flat and it falls off rapidly before end of charge. For 9V batteries, that's several volts lower than the peak (IIRC).
 

PeeSeeBee

Joined Jun 17, 2011
56
No rechargeable battery is going to have a discharge profile like that. When fully charged, the cells will be above their nominal voltage. When they discharge, there's a period when the voltage is relatively flat and it falls off rapidly before end of charge. For 9V batteries, that's several volts lower than the peak (IIRC).
If you look at the video, these type of batteries use a DC-DC converter inside the battery case to maintain 9v right up to the end. Then the battery "switches off". That's why they are not recommended for appliances such as smoke alarms, where the low battery indicator doesn't have a chance to work before the battery dies.
 

dl324

Joined Mar 30, 2015
12,442
If you look at the video
Didn't watch the video.

I generally don't waste my time on YouTube videos from so called experts. The content isn't vetted, so I treat it more an entertainment.

There have been a few times when I got useful technical information from YouTube, but that's the exception. Most of these self proclaimed experts don't know what they're talking about.
 
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