Solar powered battery charger questions - Page 1
Forum section: electronics / projects

author: diegoterc3
author_rank: Regular Contributor
timestamp: March 18, 2014, 07:28:40 am
content: Hello, I am trying to make a Solar Powered Battery charger...I have a question regarding the batteries. So I plan to charge my Ipad a 500mA and for what I´ve researching I need something like this.Solar Panel ---> dc-dc converter -----> LiPo battery charger chip for aux battery -----> Auxiliar battery ----> LiPo battery charger chip for phone ----> female usb ----> Phone BatteryI don´t know if this is actually a good idea since the auxilliar battery is supposed to charge my phone battery. I suppose if the phone battery is 5V 1A the auxilliar battery needs to be for example 6-10V 2A battery.Maybe there is a better way. I really need the auxilliar battery though because here in Lima Peru it is foggy most of the time and need to store the most ammount of energy that I can get.Also the dc-dc boost converter is used to have that constant voltage I need for the batter charger chip.I´ve watched this two videos:Anyone has any ideas? Please tell me if I am doing something wrong.Thanks

author: suicidaleggroll
author_rank: Super Contributor
timestamp: March 18, 2014, 03:09:16 pm
content: The phone interface is standard USB, which means you don't need or want a LiPo battery charger chip on that side (the phone will handle its own battery maintenance), you just want a 5v reg that's wired to the USB port properly.Don't forget a peak power tracker for your panel if you want to maximize its efficiency.

author: diegoterc3
author_rank: Regular Contributor
timestamp: March 18, 2014, 06:00:56 pm
content: Thank you suicidaleggroll!So it would be something like this.power traker ---> solar panel ---> dc-dc boost converter ---> LiPo charger chip ---> 10V - 2A battery ---> 5V regulator ---> Female standar USB right?I would like to make sure that this works sin I won´t be able to simulate it.

author: michaelym
author_rank: Regular Contributor
timestamp: March 18, 2014, 07:02:54 pm
content: I would hack an existing power bank such ashttp://www.ebay.com/itm/2600mAh-Power-Bank-Portable-External-Battery-USB-Charger-for-iPhone-5-5s-4s-YE1-/231183502132?pt=US_Cell_Phone_PDA_Batteries&hash=item35d39c4b34It has a battery+connectors+regulators for an unbeatable price, unless you want to build your own from scratch...

author: suicidaleggroll
author_rank: Super Contributor
timestamp: March 19, 2014, 01:54:28 am
content: Quote from: diegoterc3 on March 18, 2014, 06:00:56 pmYes, essentiallyMany power trackers (MPPT or other) have built in lead acid or lithium (Li-ion, LiPo, LiFePO4, etc) battery charging, so you may be able to scrap the dc/dc boost converter and LiPo charger chip and just go straight:panel -> mppt -> battery -> 5v dc/dc -> USB

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 19, 2014, 03:04:21 am
content: Start with a solar panel with a MPP close to 5V. Then add a 5.6V zener diode (or other shunt regulator) of sufficient power rating to limit the output voltage and a Schottky isolation diode. Connect that to a USB port, with plenty of bypass capacitance.As it turns out, a lot of (but not all) devices that can charge from USB will charge just fine from that. Now to ensure compatibility with a wide range of devices, add a bidirectional DC/DC converter and a rechargeable battery. Have the DC/DC controller try to hold the output at 5V, sinking or sourcing current as necessary. If the solar panel doesn't supply enough power, the DC/DC supplies the remainder from the battery. If the solar panel is supplying more power than is consumed, sink the excess into the battery if possible. The result is a much better efficiency.BTW, lithium batteries dislike being exposed to heat. Higher charge voltages only worsen the situation. For better battery life, limit the charge voltage to 4.1V/cell (instead of the normal 4.2V) and design it to keep the batteries out of the heat.

author: mark03
author_rank: Frequent Contributor
timestamp: March 19, 2014, 03:22:06 pm
content: I think the point of the "T" in MPPT is that the maximum-power point moves around, depending on the light level. Speaking of a panel "with an MPP of x volts" isn't strictly accurate.But there is MPPT, and then there is MPPT... If you spend some time looking at datasheets for DC/DC converters designed with current-limited power sources in mind, there are some which claim MPPT capability, but when you investigate more closely, they are really just using an input voltage set point and adjusting the input current to maintain that voltage. This is not true MPPT but for a small solar charger it is probably good enough, and possibly much better than the crude designs in cheap Ebay chargers.True MPPT tends to involve a lot of complexity, e.g. a processing system which periodically perturbs the input operating point (voltage or current) back and forth, to see which direction will increase the power delivery. I don't know if this capability is available yet, prepackaged into a single-chip DC/DC converter. I would be curious to find out.