What voltage battery do I need ? - Page 1
Forum section: electronics / beginners

author: Hextejas
author_rank: Frequent Contributor
timestamp: September 16, 2019, 07:37:15 am
content: This is always not clear to me so here I am again.The device that I am building has 2 sub-devices.1 needs 3.3v and the other needs 6.0vSo, do I merely sum them together = 9.3v, add in a bit of overhead, and look around for a 10v LIPO ?Or can I use a pair of DC-DC buck converters, and use them to bring a 7.3v LIPO down to the required voltages ?Thanks

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: September 16, 2019, 08:27:56 am
content: If they are just low power devices some LDOs will probably be good enough and much simpler. If you have a 6V source you could also just input 6V to the 6V device and add a regulator to the 3v3.

author: Brumby
author_rank: Unknown
timestamp: September 16, 2019, 09:22:31 am
content: Quote from: Hextejas on September 16, 2019, 07:37:15 amNo. This is absolutely wrong thinking - unless you have areallycurious circuit topology.The only sort of "adding" you do in this sort of situation is the current draw of each "sub-device".This is a more appropriate approach:Quote from: TheHolyHorse on September 16, 2019, 08:27:56 am

author: Hextejas
author_rank: Frequent Contributor
timestamp: September 16, 2019, 01:13:45 pm
content: Quote from: TheHolyHorse on September 16, 2019, 08:27:56 amThank you and what do you mean by add a regulator to the 3.3v ?

author: Brumby
author_rank: Unknown
timestamp: September 16, 2019, 01:20:29 pm
content: He means use a voltage regulator that can take a 6V input voltage and deliver 3.3V

author: hsn93
author_rank: Regular Contributor
timestamp: September 16, 2019, 01:45:59 pm
content: Quote from: Brumby on September 16, 2019, 01:20:29 pmin other words: you only need 6V source

author: radiolistener
author_rank: Super Contributor
timestamp: September 16, 2019, 07:34:42 pm
content: the better way is to use two battery. One for 6V and second for 3V. Also you can use just two 3V elements. You can get 3V from one element and 6V from both in series. In such way you can eliminate 3V regulator which will waste battery energy

author: Brumby
author_rank: Unknown
timestamp: September 17, 2019, 12:13:25 pm
content: Except that the two supplies required are 6V and3.3V.

author: tunk
author_rank: Super Contributor
timestamp: September 17, 2019, 12:28:19 pm
content: Remember that for most battery chemistries the voltage is not constant.E.g. a new AA alkaline starts at around 1.6V and then declines down tomaybe 1V depending on the application. And for most li-ions it is 4.2and 3.0V. If the voltages are critical, you need to regulate both.

author: mariush
author_rank: Super Contributor
timestamp: September 17, 2019, 12:36:07 pm
content: Quote from: Hextejas on September 16, 2019, 07:37:15 amI would say it depends on how much power each device uses and how much space you have for the battery / batteries.If the device uses little (or less) current at 6v, it may be more efficient to use a step-up (boost) regulator to boost a lower voltage (ex ~3.3v to 4v) up to 6vFor example, you may design your product with 3 AA or AAA batteries (3 rechargeable would give you ~ 3x1.1..1.3v = 3.3v-3.6v, so a small LDO would give you 3.2v..3.3v with minimal loss. Or, you may use a single 18650 lipo with 3.6v..4.2v with a small buck regulator to get 3.3vThen separately boost for 6v or whatever you need.Also consider conveniences like being able to charge the battery with a small 5v usb charger (or power the product while charger is plugged in and charging the battery)- a charger IC for a single lithium battery is less than 50 cents and works with 5v, something for higher voltage batteries is more complex.

author: mariush
author_rank: Super Contributor
timestamp: September 17, 2019, 12:41:12 pm
content: As for AA batteries, you have to keep in mind that user may insert rechargeable or plain non-rechargeable batteriesAlkaline non-rechargeable batteries will be up to around 1.65v when fully charged but they'll quickly drop to 1.5v and then go down to 1vRechargeable will be 1.35v when full and go down to around 1.1vSo let's say you design with 3 AA batteries .. you'll have a voltage range of 3v ... 4.05v / 4.95vWith 4 AA batteries, you'd have 6v with alkaline batteries, but as batteries discharge the voltage would go down quickly. So you'd still need a step-up regulator to boost 4 x 1v ... 4x1.5v to 6v

author: radiolistener
author_rank: Super Contributor
timestamp: September 17, 2019, 01:17:54 pm
content: I'm suspect that power consumption of step-up DC-DC may be even worse than linear LDO.Also step-up/step-down DC-DC have extreme high noise. It may very seriously affect device functions.While linear LDO has very clean output with no noise.In order to reduce power wasting on regulators, you can select battery voltage very close to required voltage. So the voltage difference between LDO input and output will be minimal.

author: mariush
author_rank: Super Contributor
timestamp: September 17, 2019, 02:11:46 pm
content: There are switching regulators with up to 90-95% efficiency.... it may beat the losses caused by reducing 6v+ to 3.3v... again, it depends on power consumption of 3.3v device vs 6v device power consumption ...As for noise, you can always add extra filtering (pi filters) or boost by a bit more than 6v then use a very low drop regulator to bring down to 6v

author: mzzj
author_rank: Super Contributor
timestamp: September 19, 2019, 06:08:24 am
content: Quote from: radiolistener on September 16, 2019, 07:34:42 pmProbably worth mentioning that using 2 same size batteries in series and connecting part of the load only to 3v drains one of the batteries as fast as 6v battery pack + linear regulator.You'll just end up with one empty battery and one half-empty battery..You could use two different size batteries or series-parallel combination but usually it is not worth the hassle.Nobody is happy if you need 9V battery, AA battery and AAA battery for one equipment. (seen that..)

author: Canis Dirus Leidy
author_rank: Regular Contributor
timestamp: September 19, 2019, 10:29:14 am
content: Quote from: Hextejas on September 16, 2019, 07:37:15 amBy the way. What, in fact, the requirements (size, weight, battery life, etc.) for the complete device? And then, it may turn out that one cheap 12.6V lead-acid battery and a couple of Chinese voltage converters are enough.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: September 19, 2019, 10:52:03 am
content: Quote from: mariush on September 17, 2019, 02:11:46 pmThat is true, but for someone in the beginners section who thought he should sum the voltages together to get 9.3V. He's clearly new to electronics and I don't think it's a good idea to complicate things too much.But it would be nice to know the power of the devices it would make it a lot easier for everyone to suggest appropriate things.