How do you match state of charge before paralleling two 12V lithium batteries bought at different times?

Direct answer: Use the same model and capacity, then charge each battery to full separately with a suitable charger, let both rest, and check that their resting voltages match closely before connecting them. Connect with equal-length, equal-size cables, preferably taking positive from one battery and negative from the other, and recheck after the first full cycle. Redway does not publish a maximum voltage difference, so ask for its limit for your model.

What the pages allow. The 12V 100Ah lists up to four batteries in parallel and four in series. The 12V 200Ah lists the same, and the 12V 100Ah (Ultra) lists up to four in parallel. The pages do not say whether different production years, versions (for example 50A and 100A versions of the 12V 100Ah) or different cell makers may be mixed. Ask before combining them.

Why match first. When two batteries at different voltages are connected, current flows from the higher to the lower one. With low internal resistance (the 12V 100Ah lists ≤30 mΩ) that current can be large enough to trip a BMS or stress terminals.

Why charge to full. LiFePO4 voltage stays nearly flat across much of the charge range, so similar voltages in the middle do not prove similar state of charge. At full charge the curve is steeper and both BMSs have just finished, so this is the most reliable point to match. Charge each battery separately to the page’s 14.6V ceiling or the value Redway recommends, then let them rest for a few hours.

Compare and connect. Measure each battery’s resting voltage with the same meter. If they are close, connect them with the battery switch off and loads disconnected. Use cables of the same length and size to each battery, and take the system positive from one battery and the negative from the other so that both carry similar current. Put the main fuse in the system positive as your wiring plan requires.

Check after one cycle. After the first full discharge and recharge, compare the two batteries again, and if your batteries have an app or display, compare state of charge. A battery that consistently runs out first or finishes charging much later may have lower capacity or a different version. For mixing old and new units in general, our post on paralleling rack batteries of different ages discusses the same principles at 48V.