small form factor SLA battery 18V, deck of cards size? - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: April 22, 2018, 04:58:23 am
content: Does something like this exist?For preamplifiers, so you can get nice +-15V rails, without switching regulators, or having to use a shit load of small batteries that will have sharing problems, increased leakage chance, etc.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 22, 2018, 05:22:40 am
content: Is 5S lithium close enough? How about 6S LiFePO4?

author: SeanB
author_rank: Super Contributor
timestamp: April 22, 2018, 08:13:50 am
content: Going to be really crappy capacity wise, as the plates in the 9 cells will be thin and long, and only a single one per cell, making it both very variable cell to cell capacity wise, and also making it very easy to damage by overcharging.I would agree that a 5S lithium cell pack ( or a 10S with 2 5S BMS controllers for each half) using pouch cells would fit in the required form factor size wise, and would have a decent capacity as well, plus would work almost down to the point a good LDO drops out before the BMS turns the pack off to protect the cells.

author: CopperCone
author_rank: Super Contributor
timestamp: April 22, 2018, 03:12:52 pm
content: Arent these batteries kind of a fire hazard ? Need alot of electronics to work with them too.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 22, 2018, 03:52:24 pm
content: LiFePO4 is very safe.

author: james_s
author_rank: Super Contributor
timestamp: April 22, 2018, 04:39:34 pm
content: How about NiMH? A pair of 7 cell NiMH "9V" batteries in series will get you pretty close to 18V. Well designed switching converter is the usual method used though.

author: CopperCone
author_rank: Super Contributor
timestamp: April 22, 2018, 04:41:15 pm
content: 14 batteries (i need bipolar rails) is alot.It always seems like if you get a bunch of those separate cells they tend to leak, the more the worse. And charging is a super hassle, you need to remove 14 batteries... no thanksAnd I don't want a switching converter. This is for low noise preamplifiers possibly working in the region of kHz. No matter how well designed it is, it will suck compared to a high voltage battery. I mean like parallel LT1028's or AD797's etc, for photomultipliers, preamps, portable RF equipment, oprical detectors, electrometers, other sensor boxes (microphone, etc). I tend to like to design these things to run at +-15V, which is the typical nominal datasheet value for precision op-amps. Nothing shits in my cereal more then single supply op-amp design.If I was selling something I would make a specification to some kind of noise floor and possibly use a switching converter, but I don't want to fuck with that unless I was making some kind of fat profit. I want like fast to implement R&D equipment so I can focus on the things that matter. I can see using a switching converter for these things to turn into a disaster very easily.Also how are those batteries safe? It looks like they sell complicated protection circuits for them?

author: james_s
author_rank: Super Contributor
timestamp: April 22, 2018, 04:47:34 pm
content: 14 batteries? How did you get that? I count 4 needed, the 9V batteries contain 7 cells internally but they're just the ordinary "transistor radio" battery format. A lead acid battery for 18V will have 9 cells internally, there's no way around physics.A switching converter can be very quiet. Also op-amps inherently tend to have very good power supply noise rejection.

author: CopperCone
author_rank: Super Contributor
timestamp: April 22, 2018, 04:53:13 pm
content: MIsread, 9V battery is kinda weak for running op-amps at low impedances. Also I can't really charge them in circuit, because you need to take temperature measurement to recharge NiMH cells, and I don't know how you would do it with two batteries, i would need to make a switch network to use a charger to charge them individually.

author: james_s
author_rank: Super Contributor
timestamp: April 22, 2018, 04:56:16 pm
content: You can trickle charge them in series without any measurements, a lot of battery chargers work this way. There is no internal balancing in a multi-cell NiMH pack.You could assemble your own pack using NiMH or some other cell chemistry if you like, if you want it the size of a deck of cards you are limited in your capacity by physics though.

author: The Soulman
author_rank: Super Contributor
timestamp: April 22, 2018, 05:43:34 pm
content: Quote from: CopperCone on April 22, 2018, 04:58:23 amYou can get plenty clean enough rails with a pair of 7815/7915 regulators.For mains powered application use a old fashion transformer and bridge rectifier (and caps) to feed them.Or if you need a portable solution use a 5V (usb) powerbank to power a 5V to +/- 20V dc/dc converter.Making such a small battery pack is to much hassle and longevity is mediocre at best imho even if charging and low voltage cut-offprotection is set just right.Proper grounding sequence and (local) decoupling is more effective than audiophool myths.

author: CopperCone
author_rank: Super Contributor
timestamp: April 22, 2018, 06:34:42 pm
content: what the hell are you talking about audiophool myths. I want to make precision equipment. I want battery power for a reason so that things can be portable and I can install them in remote locations for monitoring etc. you should look at a certain equipment that gets sold, it runs on batteries. why is this thread turning into pelham 123why are people coming here with crazy ass agendasif you wanna start talking in DBs and have a nice DSA, spectrum analyzer and associated preamplifiers and line conditions to back up the discussion go ahead. this thread is going in an absurd direction considering what the topic is. it should not even matter because I can put a battery pack powered device in a tree, middle of isolated field, etc. 120vac is not portable. I have a HP portable lead acid inverter pack meant for spectrum analyzers but it weighs about 30lbs. normally I power things off low noise analog lab supplies.Infact, if i wanted to use mains voltage, i would just take one of my many lab supplies and an extension cord, or my inverter, i would not even bother making a rectifier transformer shit

author: The Soulman
author_rank: Super Contributor
timestamp: April 22, 2018, 07:13:15 pm
content: By pre-amplifier I assumed audio related, didn't want to offend anyone.Your best option might be to use four "9V" nimh battery's in series for 2x 16,8V nominal and 2x21V max.So make sure your amplifier can handle that or use linear regulators.Charging nimh is a bit of a pita as they don't like overcharging but is necessary to maintain balance between individualcells as different internal leakage currents may otherwise unbalance the pack.Trickle (over) charging at 0,01C seems to be the least worse from my experience.

author: slugrustle
author_rank: Frequent Contributor
timestamp: April 22, 2018, 09:11:38 pm
content: For reference, a deck of cards is 8.89cm x 6.35cm x (about) 1.9cm for a volume of 107ml.18V is used by some power tools. You might be able to find power tool packs that suit your needs. Nice thing is, you know they test their stuff, and that the chargers and batteries are made for each other.Packs used in hobbies like RC cars / planes / drones may also be an option. Here's an 18.5V Li-Ion/Polymer pack:https://www.horizonhobby.com/helicopters/batteries/185v-5000mah-5s-30c-lipo-10awg-ec5-kxsb0026$80, 5Ah; measures 15.2cm x 4.4cm x 4.1cm for a volume of 274.21ml.Same site has a ton of chargers...Everything below is from batteryspace.com, largely because I know how to search that site.Not SLA, but as others have suggested, 5S Li-Ion:http://www.batteryspace.com/li-ion-18650-battery-18-5v-2600mah-48-1wh-7-0a-rate-battery-pack----un38-3-passed.aspx$67, 2.6Ah; measures 9.2cm x 6.8cm x 2.5cm for a volume of 156.4ml.Works with ~$40 chargerhttp://www.batteryspace.com/smartcharger15afor185vli-ionpolymerrechargeablebatterypack.aspxThe pack has protection circuitry, and the charger claims to have overvoltage cutoff, but the pack page still links to a fire retardant bag for use during charging. Not sure what to make of that.18V NiMH pack:http://www.batteryspace.com/Custom-NiMH-Battery-Pack-18V-3.8-Ah-15-x-4/3A-68.4-wh-Hump-pack.aspx$78, 3.8Ah; measures 14cm x 7cm x 3.6cm for a volume of 352.8ml. Has a thermal cutoff to prevent overcharge.Or an 18V NiCD pack made of 15x AA cells in series. Cheapest 18V thing I've found:http://www.batteryspace.com/custom-nicd-battery-pack-18v-1000-mah-15xaa.aspx$22.50, 1.0Ah; measures 7.5cm x 4.8cm x 5cm for a volume of 180ml.Edit: datasheet for AA cell in this pack is athttp://www.batteryspace.com/prod-specs/CD-AA1000.pdf. Standard charge current is 100mA, quick charge current is 300mA. The charger below is probably not suitable for this pack.Related NiMH / NiCD charger, ~$27:http://www.batteryspace.com/Multi-Current-Universal-Smart-Charger-for-9.6V---18V-NiMH/nicd-battery-pack.aspx0.9A / 1.8A selectable charge rate. Spec is athttp://www.batteryspace.com/prod-specs/2350.pdf