Help Choosing Battery Type - Page 1
Forum section: electronics / projects

author: don.r
author_rank: Frequent Contributor
timestamp: October 05, 2019, 04:57:40 pm
content: I am working on a battery backup circuit for my Sony XDR-S3HD radiohereand I have been thinking my original choice of an 18650 may be overkill. Essentially, I need a battery that will basically sit there charging 24/7 and then be needed to provide about 10uA @ 2.7-3.3V for at least a few days (power outage situation). I am thinking maybe a supercap, NiMH or NiCd might be better here. I have the 18650 already (laptop salvage) along with a holder and 1S charging board but space is rather tight inside the radio and such a big battery screams overkill to me. I have lots of charging tap points from 3.3V to 11.5V most of which can provide at least 50mA for charging.

author: Pentoad
author_rank: Regular Contributor
timestamp: October 05, 2019, 05:04:52 pm
content: I would go with the supercapacitor as leaving a lithium ion battery sitting at 4.2v all it's life is bad for it.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 05, 2019, 05:34:20 pm
content: If you want to use li-ion, just float it at around 3.7V, halving the capacity. The lifetime will very likely be far over a decade.

author: don.r
author_rank: Frequent Contributor
timestamp: October 05, 2019, 05:42:40 pm
content: Excellent. Now what are the ramifications of installing a supercap wrt charging? I suspect I would I need to add a resistor to limit the charge current?I see the 4.2V ones here:https://www.mouser.ca/ProductDetail/Vishay-BC-Components/MAL219691213E3?qs=sGAEpiMZZMuDCPMZUZ%252BYlxdFextp0taVAk4K2XCjS7c%3D

author: mzzj
author_rank: Super Contributor
timestamp: October 05, 2019, 09:33:15 pm
content: Quote from: Siwastaja on October 05, 2019, 05:34:20 pmHeck, if you happen to get good li-ion it will last in OP's use for decade even without charging!10uA, couple of days, only sometimes.Tadiran XOL in AA size would provide back-up power for 2400/88= 27 YEARS in case that the equipment is not plugged in. OP needed backup for couple of dayshttp://www.tadiranbat.com/Assume couple of days is 10 days per year, 10 years = 24mAh without ever charging the battery.CR2430 anyone? Available in every third-world country, plenty of capacity for intended use, good self-life and cost 20 cents per piece.

author: don.r
author_rank: Frequent Contributor
timestamp: October 05, 2019, 11:31:29 pm
content: Quote from: mzzj on October 05, 2019, 09:33:15 pm3V is already close to the 2.7V threshold for the mcu. Add a Schottky protection diode and we are at 2.8V.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 06, 2019, 06:27:01 am
content: Indeed, looking at the numbers, this sounds like the typical case for non-chargeable backup primary cell. Primary lithium cell, for example, because of the long shelf-life.Getting rid of charging would be a massive simplification.

author: don.r
author_rank: Frequent Contributor
timestamp: October 06, 2019, 12:32:09 pm
content: If this were a production unit maybe (but then again not for a consumer product) but not for a one off. A super cap is 2 components and $10. A LiPo is about 6 components and $4. I'll probably have to search my junk box for the solution. I'm sure I have a super cap or 2 in there and a LiPo.

author: mariush
author_rank: Super Contributor
timestamp: October 06, 2019, 04:06:16 pm
content: A CR2032 battery and a couple of diodes is cheaper.A battery holder for 2 CR2032 is 3$ and CR2032 batteries are super cheap, like 4 for 1$ or something like that. Add a couple of diodes to drop some voltage (below 5.5v) and you're in business.If you go with super capacitors, you can get 2.7v rated ones. In the 18650 footprint, you can squeeze 100-120 Farad capacitors.If you want smaller, you could do for example 47F in 18mm x 33mm :https://www.digikey.com/product-detail/en/nichicon/JUWT1476MHD/493-4338-ND/2538692You can use a 2.5v LDO to make sure you don't overcharge it, and then use voltage doubler to get up to 2x the voltage out of the supercap ...See for example the 11 cents MIC5504 :https://www.digikey.com/product-detail/en/microchip-technology/MIC5504-2.5YM5-TR/576-4880-1-ND/5209408It's a 300mA LDO, with a current limit at around 600mA (min 400mA, typ 630mA, 900mA max) so it would probably be safe to use with that other regulator (if the supercap is fully discharged, you don't want a bigger LDO that would suddenly suck 1-2 A of current to charge the supercap.Downside is the supercap (if it's big size) will probably take a few minutes to charge up to 2.5v but in your context this isn't an issue.For voltage doubler, see for example NCP1729 :https://www.digikey.com/product-detail/en/on-semiconductor/NCP1729SN35T1G/NCP1729SN35T1GOSCT-ND/1967221It does have a current of around 130uA, but you can use the energy of a supercap from 2.5v down to maybe less than 1.5v.See page 11 of the datasheet for example circuit.Alternatively, you can buy a 1-2$ step up regulator to 3v or 3.3v or 5vHere's an example at 2$ :https://www.ebay.com/itm/3V-3-3V-5V-DC-DC-Boost-Converter-step-up-Voltage-Regulator-Power-Supply-Module/272598733583They work from 0.7v and upYou could also use one of these regulators with a plain AA or AAA battery. Even non-rechargeable ones would last months in it, at 0.1mA or less current.You could even go with a crappy NiCD battery and trickle charge it with 1.25v.. 1.45v using a fixed voltage ldo, see for example ablic s817 serieshttps://www.digikey.com/product-detail/en/ablic-u-s-a-inc/S-817A13APF-CUCTFU/1662-2680-1-ND/7228972https://www.digikey.com/product-detail/en/ablic-u-s-a-inc/S-817B13AMC-CWCT2U/1662-3264-1-ND/8022396or NCP512:https://www.digikey.com/product-detail/en/on-semiconductor/NCP512SQ13T2G/NCP512SQ13T2GOSCT-ND/2683394

author: don.r
author_rank: Frequent Contributor
timestamp: October 06, 2019, 04:55:50 pm
content: Its probably going to come down to what I have on hand. Not a project I wish to invest a lot of time and money in and I would like to K.I.S.S. since I don't want to have to disassemble the whole radio to repair it (although I have other mods planned for this thing).

author: mzzj
author_rank: Super Contributor
timestamp: October 07, 2019, 07:06:12 am
content: Quote from: don.r on October 06, 2019, 04:55:50 pm18650 Li-ion probably works for your use +10years without charging."Probably" because they are not specified for this..