How to replace 9V battery with 1.2V Cell? - Page 1
Forum section: electronics / beginners

author: 001
author_rank: Super Contributor
timestamp: November 06, 2018, 03:27:44 pm
content: HiI want to replace 9V batteries in my gear with 1.2 NiCd cell + step up converterIs anybody goes this way before?

author: glarsson
author_rank: Frequent Contributor
timestamp: November 06, 2018, 03:38:44 pm
content: The original Texas Instruments TI-30 from the late 1970:ies could use a 9V battery or a rechargeable battery. The rechargeable battery was one single NiCd cell, a step up converter and a 9V battery connector. Perhaps someone can find a schematics.

author: johnkenyon
author_rank: Regular Contributor
timestamp: November 06, 2018, 05:25:06 pm
content: Why not go for two 1.2 NiMh cells and a MT3608 boost converter (works down to 2V).

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 06, 2018, 05:43:26 pm
content: why not to got for 9V block with a LiPo cell and integrated charger?

author: 001
author_rank: Super Contributor
timestamp: November 06, 2018, 06:50:48 pm
content: What do You mean?

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 06, 2018, 07:51:51 pm
content: yep, those ones; they're really practical, and last much longer than the usual 9V NiMh

author: mariush
author_rank: Super Contributor
timestamp: November 06, 2018, 08:41:45 pm
content: Figure out what's the minimum voltage your gear will run at before showing "low voltage" or before its performance goes bad.A lot of products use linear regulators or dc-dc converters to take the voltage from battery down to something lower like 3.3v or 5v, and the input voltage in most cases only needs to be around 0.5v to 1v above the internal voltage for everything to work right.For example, my Uni-t UT61E will only show low voltage when battery goes down to around 5.6vNow you can use a step-up (or boost regulator to raise the voltage from your battery up to the desired voltage. Typically, the higher the voltage (the difference between battery voltage and output voltage) the less efficient the converter will be, so it's in your best interest to only boost the voltage just a bit above that threshold where the product would say "low battery" or go wonky.For example, in the case of my Uni-T UT61E multimeter, it will work just as well with 6v or 9v, so I'd configure my step-up dc-dc converter to raise the voltage to around 5.8v (a decent 0.2v above the minimum voltage, allowing for 50-100mV ripple on the output)You can buy all kinds of step-up dc-dc converters, eBay is full of cheap 1-2$ tiny circuit boards that can do that.Just keep in mind that some will work with voltages as low as 0.6v or something like that, but they'll only be able to output a limited amount of current in such scenario. For example, a chip may only be able to do 100-200mA with 1v input and 3.3v output. But, it may do 500mA or more with 2v input an 3.3v or 5v output. So may be better to have 2 batteries in series or a lithium battery instead of a single battery.You should focus a bit on the battery as well ... NiCd may be easy and idiot proof to charge but in terms of energy per volume it's not great.You could try going for 1 or 2 AAA alkaline batteries or a single lithium battery that would have higher voltage from the start (~3.7v ... 4.2v)

author: 001
author_rank: Super Contributor
timestamp: November 06, 2018, 09:13:49 pm
content: ThanxThe reasons to not to use Litium is winter and selfdischargeThe main problem for me is how to build DC-DC converter that will be active when DMM is on only?

author: mariush
author_rank: Super Contributor
timestamp: November 06, 2018, 09:59:20 pm
content: I guess you could pick a step-up regulator that has an ENable pin , which must be either connected to input voltage or to ground to turn on the regulatorthen, figure out how your meter turns on (maybe there's two traces on the circuit board under that rotary wheel used to select ac voltage / dc voltage / resistance etc and you can just solder a wire to one of those traces.OR, you could just program a 4-6 pin microcontroller (for example a 20-50 cent PIC10 or ATTiny) which would simply run 24/7 from battery using uA of energy and waiting for you to push a power on button ... when the button is pushed once, the micro can send voltage to the enable pin or connect the enable pin to ground and turn on the regulator.... then wait some time (for example 30minutes) and then turn off the regulatoror you could just use a modern step-up regulator with very low quiescent current, like 10-50uA, and just leave it running 24/7 - if the meter doesn't run, the discharge rate would be super low.For example, there's MAX859 .. 0.8v .. 6v to maximum 6v output at up to 125mA :https://www.digikey.com/product-detail/en/maxim-integrated/MAX859CSA/MAX859CSA-ND/1513203it has a 25uA quiescent current, or 1uA in shut down modeTheMAX856version that runs at higher frequencies and can do up to 0.5A output, stays at around 50uA quiescent current as long as the input voltage is reasonable (like 2.5v or higher, typical for 2 rechargeable NiMH batteries or alkaline batteries)Another example would beTPS61220... up to 6v output voltage with high efficiency and very low quiescent currents ..datasheet says typical 5uA

author: glarsson
author_rank: Frequent Contributor
timestamp: November 06, 2018, 10:26:41 pm
content: Quote from: 001 on November 06, 2018, 09:13:49 pmThe self discharge of NiCd (your proposed cell type) is absolutely terrible compared to any lithium type cell. Can you even find NiCd today?

author: mvs
author_rank: Frequent Contributor
timestamp: November 06, 2018, 11:49:48 pm
content: Quote from: mariush on November 06, 2018, 09:59:20 pmTI makes a bit marketing with numbers here5µA is quiescent current that flows throught Vout pin (internals of ultra low voltage step up converters are usually powered from Vout rail).To get idea about real current consumption from battery, one should use Figure 8 of the Datasheet. And even then, TI have not used any feedback divider for TPS61220.

author: thm_w
author_rank: Super Contributor
timestamp: November 06, 2018, 11:58:55 pm
content: The lithium 9V are incredibly good, at least from what I've seen in my DMM/LCR meter.Also you don't need to get one with an integrated charger if you have an old 9V battery charger. Some are available that you trickle charge, then when it hits ~8.4V it will shunt the charge internally. So just use a low current charger (fast charge would probably blow the thing up). But USB is convenient too.https://www.aliexpress.com/item/2pcs-Soshine-Power-Battery-6F22-9V-Li-ion-Lithium-650mAh-Chemistry-Rechargeable-Battery-For-Electronic-Instruments/32844425337.htmlFor comparison a 9V nimh is 200-250mAh.

author: malagas_on_fire
author_rank: Frequent Contributor
timestamp: November 07, 2018, 12:19:39 am
content: The NiMH 9V batteries or rated 8.4V although have low capacity as 220mAh they quite accept well the trickle charge from that old chargers, lets say 22mA to 44mA, doesn't need special transport as lithium batteries supposelly required as well safety, but looses when demanding current and capacity are required.Watch out for temperature range of the appliance lithium doesn't like too much heat has well.About the boost circuit remember the teardown of the 9V batteries from daveOne of them has multiple cells of 1.2 V while the other is a solid cell.