I am new to the DIY solar, never knew that batteries in series you cant use a BMS, How would you manage batteries in series?
You
can use them in series, but under certain conditions. First, under the assumption that you plan to charge the battery as one big battery, the individual batteries must have BMSes that support series battery-connections and the possible high voltages they may be subjected to in such conditions. Usually the battery listing will inform you of what they support.
Second, you need to deal with balancing. Just like the cells in a battery can become unbalanced - affecting performance and causing degradation -the batteries themselves can become unbalanced between each other. The BMSes handle inter-cell balancing, but batteries will not be aware of each other's state.
To help with battery balance, you'll generally want the batteries to have similar characteristics so that they operate somewhat synchronously (same discharge characteristics, voltages, etc.),. This can mean connecting multiple identical batteries purchased from the same seller at the same time, to increase the likelihood that the cells came from the same batch and use the same BMSes (or testing their characteristics to be sure). That's usually a context where series-connected batteries are depicted in battery sale listings.
On top of that, you will probably want to add an inter-battery balancer can help compensate for when they inevitably drift - but don't depend on it to compensate for batteries with very different characteristics. When hooking them up, the voltages should be the same too.
If you're not planning to charge the batteries as one big battery, you can also charge the individual batteries separately and hook them up together in series if you need a higher voltage bank. That can be a nice option for, for example, charging 12V batteries separately and connecting them together for higher voltage needed for a golf cart or something. Some people will still charge the whole bank at once, but avoid the inter-battery balancer, doing a manual rebalancing every 6-mo or so.
So check out other posts on the forum about this topic if you want some best practices.
With all the conditions above, if you're buying new, it's probably better to just purchase a single battery at your target voltage than cobbling together a bunch of lower-voltage batteries and adding a balancer (or multiple). Still, you can weigh your options, because sometimes multiple batteries + balancers is more economical than a single large battery if you have the space and time to do it right!
