This video is a bit more common to see when people are building or storing battery packs while getting ready for solar or they have solar but need to address the battery SOC.
The battery pack in question was not a solar installation battery as it was a Ultramax 40ah LFP battery for a mobility scooter, there a pair of batteries which just fit in and well much lighter than lead acid. The problem is that I do have a charger for lead batteries but it has a dissulfate function, meaning that it cannot be used, Charing a lithium battery should be CC CV. That’s Constant current, Constant voltage.
In steps the “bomb” it got its nick name from an event that took place a year or so ago when someone broke there internet routers power brick and i was tasked with “can you help”. so grabbing a few bits, and ten minutes later we have a 12v 2amp power supply and the internet was all working again. I have long cables so they were wrapped and taped up and it well… looked like a bomb.
NOTE: This is not a safe way to charge a battery and has a lot of hazzards so, don’t try this at home!
How to make a Charger bomb…. ( just a charger)
The charge bomb is basically things that you may have laying about, a old laptop charger a DC buck converter, some cables, a voltage and current meter, ring terminals… tape. Then a diode 10amp and some 13amp block terminals and a 10amp micro automotive fuse.
A laptop power supply is around 20v, and you need to pay attention as some of the new ones are low current as tech evolved the power requirements reduced, so you need to check what the power is. HP and Dell laptops from the early 2000s would be best and to give you some idea, the one here is a Dell laptop charger 21v 6.5 amps, Most of the Lenovo laptop chargers are around 19v and 3amps, so effectively half the power.
If you recall many of my posts on converting power you will know about power factoring (ohms law). We need to start at the point of what is the charger source voltage, and what is the output voltage (charge voltage) if we are going down in voltage we buck the power, if we go up in voltage we boost the power.
this means as we go down in voltage, we go up in current and the opposite applies to going up in voltage.
The power pack will say its amps and watts max, so we know we do not want to exceed this, or we do end up making a bomb. The dell psu with its maths we know that we have 136.5watts of power, but the sticker says 120w max…
This means that you can have 120w into the battery or cells, or 10 amp charger…..max
Getting the right voltage
Now that we have a power source, the voltage is higher than what we need, so we need to buck the voltage down.
A LFP cell has a charge voltage of 3.65v this is a 12v battery and has four cells, so we need to have a charge voltage of 14.6 volts
21v / 14.6 = 1.438 (pf)
6.5a x 1.438 = 9.349a (oa)
We take our buck converter, which has CC CV and adjust the pot ( blue thing) to change the voltage to 14.6 on the output, we also wind in the current as we want to start low.
The power meter we used did fail, so we would use this one from the shop – it will save you some math and is less confusing to use.
PROTECTION:
In order to correctly build this, you should have a fuse and a diode, in the event of a fault in any of the parts, you will want to be protected so your battery does not back feed its entire energy to the fault.
If you are using a straight diode, then you can wire this to the terminal, on the other end put a block terminal, On the other side, you place the fuse to bridge the two block connectors and then wire out to the source. if you’re using the meter from the shop.
Then you have reverse protection and overload protection in a simple format.
Ensure that you have good wire to pass the power though, we tend to make chargers and use silicone cables, if you are stuck power cable from a psu/ kettle can be used as this is normally rated at 16amps. for our 110v users ensure that your not using .75mm2 or less cables as this can heat up, if it looks thicker than speaker cable you should be good to go. – DO NOT EXCEED Output cables current output and limit current with a fuse.
With the power meter you can see the voltage, amps being passed to the battery, with the monitor it does show the AH and Wh which if you know your battery capacity in either or both of these, it can be very helpful to monitor the SOC of the battery and time until it is charge.
The Buck converter has a LED, which will turn blue or green when charging (current flow) and red when holding constant voltage (charged).
Do not exceed 8 amps output, as this buck is the 200w one. and you need to keep it cool, hence the fan, you can buy bigger and bucks with fans a 50mm fan can be screwed to the heatsinks and powered from the output. as you can see we used a room fan as we have a hot day here in the UK.
Turn on power supply before connecting to the battery!
Always monitor the charge!
We would recommend only charging at 5 amps
RenewSolar make battery chargers and auto chargers which are safe and not a ghetto bomb type charger. But this can get you out of a bind if your stuck without the right charger and you need to get things done. Ensure you fuse and work well within the limits of the hardware.
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