charging my phone from my ebike battery - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: September 15, 2021, 10:27:14 pm
content: Hi all,my ebike is 13s 48v nominal,i want to be able to take a 5v feed from it to charge my phone as my smart bms uses my phones gps to calculate remaining range etc,48/54volt from the battery is to high for a 78 series of regulator tho,any ideas how i can do it?,i could spur off a couple of cell groups but i want to avoid that to keep the cell groups in ballance,thanks in advance,i only need about 500ma at 5v.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 16, 2021, 01:20:01 am
content: A PoE (Power over Ethernet) to USB adapter is a commonly available 48V to 5V converter.

author: m3vuv
author_rank: Super Contributor
timestamp: September 16, 2021, 01:34:58 am
content: would a zenner and a pass transistor work i wonder?

author: viperidae
author_rank: Frequent Contributor
timestamp: September 16, 2021, 01:40:16 am
content: I second the opinion of a Poe adapter. They're designed to accept up to 56v usually and are isolated too. The last thing you want is 54V going to your phone if the controller or switching transistor fails.You won't need a full on Poe adapter either, a "passive" one is probably better as it doesnt include any Poe negotiation.

author: james_s
author_rank: Super Contributor
timestamp: September 16, 2021, 03:48:30 am
content: LM2576HVT will work. The HVT suffix is 60V max input, all you need is an inductor and a few passives to build a regulator putting out whatever voltage you want.You absolutely MUST use some kind of switching regulator for this application if you want anything even slightly efficient. Dropping 50+V across any kind of linear regulator at more than a few milliamps is going to result in a massive amount of wasted power. Linear regulators all behave like a controllable resistor, all the extra voltage is just burned off as heat.

author: m3vuv
author_rank: Super Contributor
timestamp: September 16, 2021, 03:09:41 pm
content: well i just got back from a 20 mile ride,cell bank number 3 is 200mv lower than the rest so may pull the battery apart and add 2 more cells to bank 3 and take a spur of say 12v and use a 7805,,luckily theres room for another couple of cells,there are 117 at the moment.

author: m3vuv
author_rank: Super Contributor
timestamp: September 16, 2021, 04:30:15 pm
content: well i looked at the digikey dcdc converters,well the uk store£12 for the modual,almost £13 shipping,they can stick it where the sun dont shine,what a total ripp off,may get one from ebay uk for about £8!!

author: james_s
author_rank: Super Contributor
timestamp: September 16, 2021, 09:13:03 pm
content: Why are you so fixated on the 7805? It's a completely obsolete part in most cases, unless you have one of a very few niche applications, charging a phone is not one of them. It would be absolutely stupid to use a linear regulator for this, there's a name for that sort of thing, "penny wise and pound foolish." £12 does not strike me as very much money, it could easily cost that much to get lunch. £8 is even less, in the context of what an e-bike costs that's nothing. If something is worth doing, it's worth doing right.

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2021, 10:04:18 am
content: Quote from: m3vuv on September 16, 2021, 04:30:15 pmI don't think £25 is an unreasonable amount to spend, compared to the price of the batteries, bike and phone. It might end up costing you more in the long run, if it blows up and destroys your phone.If you must go for a cheap power supply, choose one with a much higher current rated, than you need, say 1A minimum. Fuse the input and consider adding an over-voltage crowbar and fuse to the output, so your phone doesn't go up in smoke, if it decides to fail and dump the full 48V into your phone.

author: m3vuv
author_rank: Super Contributor
timestamp: September 17, 2021, 01:31:24 pm
content: think i sorted it,had a solar phone charger,it was a strait output from the solar panel to the charge cable via a 1n4007 diode,i replaced that with 5 x 1n5711 diodes in parellel,seems to be working fine.,dont know why i didnt think of this before.

author: james_s
author_rank: Super Contributor
timestamp: September 17, 2021, 08:26:11 pm
content: What purpose are the 5 diodes in parallel? Do you mean you're powering the solar charger from the bike battery instead of the panel, and you're expecting it to handle greater current because you added more diodes? If that's the case be sure to post some pictures of the aftermath when it blows up and destroys your phone.

author: m3vuv
author_rank: Super Contributor
timestamp: September 17, 2021, 10:46:00 pm
content: i am using the small solar panel to keep my phone toped up,while using the bms app and a cycling gps app,the diodes are in parelell to handle more charge current the are shkottys so only .3v drop,the original diode on the panel was a 1n4007 silicon so .6v drop,the panel only outputs 400 ma tops.