# Voltage vs Battery Life trade off

I have a PCB board which is supposed to be powered of 12V supply.However, now I would like to run this of a battery.

Most LiPo batteries are 3.7V , and 12V batteries are big and hard to get. What is a effcient way to boost this 3.7V to 12V making sure this voltage conversion has maximum efficiency?

I am looking basically at http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF . But converting 3.7V to 12V must be something that is very common. Is there any IC which does the charging and conversion from the LiPo in one package? Something that monitors the amount of electrons in a battery and regulates the current for a longer life(like the IC circuitry on a cell phone)

Also I was looking at Li-ion va lipo here http://www.androidauthority.com/lithium-ion-vs-lithium-polymer-whats-the-difference-27608/ and it says the cycles is 100-400. I thought mobile phone batteries have a much longer life? Any suggestions?

At 5V, I am expecting about 1A to be drawn by the PCB.

In order to be safe I am expecting about 500-600ma to be drawn from this board. Tolerance is not really important.

• "At 5V, I am expecting about 1A to be drawn by the PCB" - if it draws 1A at 5V then why does it need 12V? Mar 13, 2016 at 2:12
• @BruceAbbott, I agree a confusing statement. What I meant was I have a very similar board which same IC and this draws about 1A at 5V Mar 13, 2016 at 2:15
• Perhaps you should tell us more about this board. How much current does it draw at 12V? How stable and accurate does the supply voltage need to be (max/min volts, ripple?). How long do you want to run it on battery power, and what is an acceptable recharge time? Mar 13, 2016 at 4:12
• @BruceAbbott about 5-6 hours. The power is actually really tough to estimate since it is not 500-600ma all the time. ANyway to first do a power measurement cheap? Mar 13, 2016 at 4:43
• Looking at the multimeter is one, but running it actually for 6 hours and see how much power is consumed is another Mar 13, 2016 at 4:44

If your board draws 600mA at 12V then the boost converter needs to output 12*0.6 = 7.2W. Assuming 80% efficiency the input power will be 9W, which is 2.4A at 3.7V. So the converter has to draw 4 times more current from the battery at 3.7V than it delivers to the load at 12V.

Peak switching current will be higher because a boost converter cannot run 100% duty cycle. At 90% duty cycle you would need 2.4/0.9 = 2.7A. The MC34063 can only switch 1.5A, so it is not suitable unless you add an external power transistor.

Typical Lipo cycle life is 100-500 cycles depending on current draw. However this is for 100% charge/discharge. Cell phones may limit charging and discharging to less than full capacity in order to extend lifespan. As the battery ages its internal resistance increases. This is more of a problem at high discharge rates, so a device that goes for several hours on a charge could get much longer lifespan than one which drains the battery in 10 minutes.

To calculate the battery capacity required you first need to know the average current draw. For example if the board draws 600mA 25% of the time and 200mA the rest of the time, the average current draw is 0.6*0.25+0.2*0.75 = 0.3A. Now translate that to battery current at 3.7V; 0.3*4 = 1.2A. To get 5 hours run time the battery would need to have a usable capacity of at least 1.2*5 = 6Ah (6000mAh).

If the average current is very low then a much smaller battery could be used, but the boost converter must also have low quiescent current draw. Most modern boost chips have very low Iq, but some older types are quite high. Manufacturer and supplier websites often have a parametric search function that you can use to select a suitable part, eg. Texas Instruments Step Up (boost) converters.

I don't know of any IC that charges a single lipo cell and boosts it to 12V. However there are several that boost to 5V. Is it possible that your board only uses 5V internally? If so then perhaps you could bypass its on-board regulator and power it with 5V.

Another option might be to use a 3S Lipo, which has a usable voltage range of 11~12V. Then you won't need a voltage booster. However you may want to include a method of checking and/or maintaining cell balance. Under/over voltage and over-current cutoff would also be a good idea. Protection circuit boards which do all that are readily available, or you could get a battery which has built in protection.