Use a tender charger from HF (link).

https://www.harborfreight.com/automatic-battery-float-charger-64284.html

which is now $10, I bought several years ago for $5. The output dc is 15 V, 0.4 A. Wait until they have 30% off on one item.

Or use a module of 38V 6A DC-DC Converter Digital Boost Buck Step Up Step Down Module CC and CV

$22.22+$3.5 shipping, (try to find a cheaper one than this, I bought several years ago for under $15 with free shipping. Or go to aliexpress). You have to have your own power supply which has higher voltage than the voltage you require. The advantage of this module is constant voltage and constant current which is perfect for lithium-iron battery. 

https://www.ebay.com/itm/182410262847?_trkparms=ispr%3D1&hash=item2a787fb93f:g:tlQAAOSw5BpdLWqg&amdata=enc%3AAQAGAAAA8IieHi0QjIH0os%2BR5rdVbbWBlu%2FyMJZIlrSxYy4WRCSfhpeLASwLofZIrST20Nfkyp7zT%2Fk%2BIQnqUVYCaiHiVOAfrSJFaaShqAZzB9iERnowqUzXmcwr%2FuyGm0I%2BJJH%2Bgd7uUSkL0vqVYTORhrDL%2BcSBb%2BFAfOblQyOA9YGZZRDapz5bg7lUJeFU%2BtcWKaftQG9gk7OyOEJx%2Bw069HWuOmdSrzt3UP3vl8hnPAZw7U64M7LJkWdh6XmXK7kbZkWy5YYFhsAwdOKw%2FG1CMD6TNCHNaR3zmzEM5xa5hisO1zcahsor1vjPs1d3pxfHhV%2Fr3A%3D%3D%7Ctkp%3ABFBMuOm0_oZg