Since I’m starting to use more 9V rechargeable batteries, I thought it would be a good idea to build a simple 9V rechargeable battery for my friendly guitarist instead of having to urge him to look for and gather bunch of expensive 9V batteries day-to -day. The simplest way would be to make it with a small 1S (3.7V) lithium polymer (LiPo) battery and then use a minuscule dc-dc boost converter module set to 9VDC output. It may not be the smartest way, but it works. Let’s get started!
Compact 9V battery holders and 9V battery cartridges are very typical things found in every guitar store as those accessories are commonly used in active bass guitars and guitar pedals, etc. In some cases, two 9V batteries are used together in parallel to prolong the playtime, therefore special dual-9V battery holders/cartridges are also available.
First, what’s active bass? The term active bass usually refers to the addition of a preamp powered by a battery. Most active basses have passive pickups with active circuits so that the electrical signal comes out of the passive pickup runs into a powered preamplifier. For more on active bass, you may go to https://www.talkingbass.net.
Below you can see the battery room of a typical active bass guitar.
As you’ve probably guessed, the common 6F22 9Vbattery is not a strong one, so naturally when it gets to die soon the guitarist on stage starts to face serious issues in the sound. Replacing the traditional 6F22 9V type with a stronger rechargeable 9V battery/batteries can hopefully clear up that mishap.
Coming to the surface, even the smallest lithium-ion battery pack won’t work for this because an ultra-thin battery pack is crucial for the proposed project. Opt for a 1S (3.7V)/500mAh or 600mAh lithium-polymer (LiPo) battery (commonly used in remote controlled toys).
It’s worth noting that a lithium polymer battery is sensitive to both under and over voltage charging and can catch fire or explode if overheated or short circuited. Most LiPo batteries are sold with integrated protection circuits. In principle, the tiny circuit board secured by Kapton® tape will render decent protection against overcharge, deep discharge, overcurrent (charging and discharging), and accidental short circuit.
Abiding by commercial aspect next needy thing is a 1S LiPo battery charger circuit/module that can charge the battery from a USB standard 5VDC power supply. You can try the omnipresent TP4056 Li-ion/LiPo battery charger module, but it is not advised here.
It’s simply because some other circuit/module is also required to boost the LiPo battery output (3.7V-4.2V) to 9V, and using two separate circuits/modules is possible (see the primal block diagram) but too impractical for a construct which must be small enough to fit inside a little enclosure. Let’s find something more sensible!
Across the web there are countless modules suitable for my o it yourself 9V battery idea. The easiest way to get off the ground is to buy the very popular and utterly cheap version I bought from Banggood. The electronics of the minuscule module is a 1S Li-ion/LiPo battery charger circuitry followed by an adjustable DC-DC boost regulator circuitry. Simply Google “HW-357 (v3.0) module” to grab it from your best-loved online seller. Or go through this link https://www.alibaba.com/product-detail/HW-357-3-7V-liter-5V9V_62295137412.html
In this module, the TC4056A chip (http://www.datasheet-pdf.com/PDF/TC4056A-Datasheet-FUMANELECTRONICS-1309136) is used in the battery charger circuit. The charger circuit is configured for 1A charging current but that can be reconfigured by changing a single resistor (122) wired to pin 2 of TC4056A if necessary.
The boost converter is based on B6289Y chip, which is a fixed frequency, current mode boost converter in SOT23-6 package. The B6289Y includes soft start, input undervoltage lockout, current limit, and thermal protection. See its key features:
- Integrated 80mΩ power MOSFET
- Wide input voltage ranges from 2V to 24V
- 2MHz fixed frequency operation
- Internal 4A current limit
- Adjustable output voltage
- Chip internal compensation, simplify the total number of external components
- The output voltage supports up to 28V
- High efficiency: up to 97%
- Automatic PFM (Automatically switches to PFM mode at light load)
This annotated image will help to make the construction an easy job. Beware, incorrect wiring will kill the module immediately!
Now how to use the battery in practice. You can wire the circuit as pointed to fit it complete with the LiPo battery in a suitable enclosure.
Since I couldn’t find a type of enclosure which is readily available and has a built-in connector socket, I bought a couple of 9V battery boxes and holders, and the minimal idea has been tried with delighting results. In order to use the inbuilt micro USB socket for charging the battery, it is essential to make a cut in the enclosure at the right point – not hard, but a steady hand and a sharp knife are required.
And now – I finished my initial experiments over the last weekend, and well look below. Although the assembly was originally put together quickly as a proof-of-concept, it has been in use for quite some time. While, I would recommend that the circuit should be made in a safer way and in a better enclosure (Perhaps a 3DP one – then battery contacts can be taken from a discarded 9V battery or a 9V battery snap connector) if you intend to make several’ copies’ for sale at a time. It’s up to you!
Here is a 3D printed enclosure for a do it yourself 6F22 9V rechargeable battery project. This is the download link https://cults3d.com/en/users/sign-in. You need to sign-in or sign-up before downloading. Be patient.
Although the dc-dc boost converter chip has the part number B6289Y, its wiring closely resembles the typical application circuit of MT3608 (Aerosemi) 1.2MHz, 2A, Step-up Converter. Perhaps, the chip might be another make but with similar electrical characteristics (sees good).
Also, it appears that the scheme of course brooks from the charger circuit output going to the battery and the battery output (via its internal protection circuit) going to the step-up circuit. If the battery is flat, and the external USB charger is connected, the charger circuit will not only have to charge the battery but also handle the booster circuit– time will tell how cool that plays!
Moreover, it’s quite true, the boost (step up) converter circuit introduces noise and ripple. Although noise and ripple are not of concern for basic use, they are very important in audio-related applications. You must take a test to measure the output ripple voltage under load. I’ve not yet conducted the ripple test. I got a little lazy, so more on that later…
The next thing to do is the design of a low-battery indicator circuit for use with the homemade rechargeable 6F22 9V battery tailored for pickups and other electronics that run on a 9V battery. An LED indicator in the proposed design flashes when battery voltage falls below 7.7V. This early warning notifies the player it’s time for a recharge well before tone begins to suffer.
I just started the work, and now eagerly waiting for a devoted CMOS voltage detector IC from abroad to arrive in the next few weeks. See you again soon!