If I charge 6 li-ion cells Which I believe would be 22.2v with a constant 18v would that be ok do you think?
Not quite sure what you are saying there. For a simple charge/discharge regime with six Lithium Ion (LiIon) cells in series you need 4 * 6 =24V constant voltage with a current limit of 1 * C say (C is the the battery capacity, say 3.4 A/H). Any lower current limit would also be suitable.
A conservative cutoff voltage would be 6 * 3 = 18V.
To make a suitable LiIon battery pack you would connect six LiIon cells in series and also connect a battery balancing and protection circuit to the six cells in series. The parts to do this are reasonably cheap and widely available.
How to connect the batteries in series? There are a few practical options:
(1) Buy batteries with leads attached
(2) Ask the battery vendor to attach the leads for you
(3) Use battery holders (as previously mentioned).
In my view (3) is by far the simplest approach, provided, of course, that you have room for the battery holders (they are not very big):
https://www.ebay.co.uk/itm/Batterie...721376?hash=item1a1c3a2160:g:c5kAAOSw8w1X4NGD
Here is a link for a 6 cell LiIon battery balancer, protector and cut-off circuit: **broken link removed**
Don't worry about LiIon batteries. They are normally trouble free and simple to use. They are employed all over the place from mobile phones, thru cameras, thru power tools, through electric wheel chairs, to electric vehicles, typified by Tesla cars, which use LiIon 18650 cells and a conservative charge/cut-off regime.
The reason for LiIon batteries wide use is that no other commonly available battery can match them. They have around 2.5x the energy volume and x 0.33 the weight for a given energy storage. LiIon batteries also have the best energy retention of all commonly available high-power rechargeable batteries. They also have a relatively stable voltage and low internal resistance throughout their discharge cycle and you can use most of the energy stored, within a particular regime. The other major advantage of LiIon batteries is that they have a charging efficiency approaching 100% compared to 50% for some other battery types. This means faster charging and less wasted energy.
This is not a high current drain application, so LiPo batteries are not necessary. Genuine LiPo batteries have a lower nominal voltage of 3.4V, which will complicate the issue anyway.
Just a word of caution, only use mainline manufacturer's batteries (Panasonic, Sanyo, Samsung, LG, Sony) from a a reliable source. Don't be tempted to buy batteries that appear to be cheap and high performance- they are a rip off. This is the supplier I would recommend to get decent batteries at a reasonable price:
https://batteriesplus.co.uk/acatalog/copy_of_Lithium_Ion__Li-Ion___3.7V_Batteries-1.html
The only problem I can see with this project is the volume available to house the battery pack. Can you please specify this?
spec